So I've been offering this service since December and it is by far my most popular enhancement being added outside of my core elements of lighting and photo booth, with 17/60 main events (weddings/sweet16s) adding it on in 2 or 4 unit variations. That number will probably rise, as many couples that have booked me for 2019 did not have this option available at the time of booking, and as I present it during the finalization with a special introductory rate, many are adding it then. Currently 8/24 weddings have this added for 2020, and likewise more will add it as we get closer to the date of the event. For NJ couples, many are already exposed to this as most NJ dj's offer it in some form, but for my PA couples who aren't exposed to it, my photos and videos I've accumulated from my initial events have done an amazing job selling this option.
Now as far as who sells it... the most well known and best company for this (although most expensive), is Sparkular. Their units are pricey... at nearly $5000 for your first two units and the controller, and then additional costs for the granule packs. You do need their proprietary controller as thats what monitors the units for heating and error cords, as well as make adjustments to the heights and programming. I believe you can do it through a dmx controller too, but you won't have any monitoring capabilities, and for insurance reasons (which I mention below), you need to be using their controller. I have 2 of these units, and have a friend who also invested in 2 that I rent when I need 4. The spark effect, in my opinion, also look the best from this company and goes highest for tall ballrooms.
The additional benefits of this company are that all units are field tested to ensure product reliability. They also come with liability insurance directly from the manufacturer. Many of the typical dj insurances wouldn't cover this effect... but Sparkular does insure their units should something happen at an event you're using them at. All units sold require the user to take and pass an online certification course. There is a database available of all the people that have been certified in case a venue or fire marshalls calls them. They work directly with fire marshalls all over the country to ensure them that their products are safe and the user is certified. In NJ this isn't so much an issue with the marshalls as these units are used by many all over, but in less familiar areas this may be a big bonus to avoiding conflicts.
There are additional companies out there, mostly all knock offs sold online via ebay or amazon for a fraction of the price. They don't have any monitoring abilities, and typically the units aren't as well made, with many reporting overheating or blowers getting stuck. In America, PROX sells the Blitzz units, which I have 2 units of personally, and bought prior to the Sparkular units. The blitzz are a great build and work very well in my opinion. They don't go nearly as high as the sparkular but still good enough. Brand new, they are about $3000 for 2 units and they come with a remote control and a road case. Granules are extra of course. In my opnion, they're the same as the chinese knock offs, just rebranded... kind of how adj and chauvet often appear to be rebranded chinese made products. However, with prox you get the warranty and support. One of my units had an issue and prox offered to pay for shipping costs to get it fixed (just needed to be updated so the two units shot the granules in the same manner, as one shot lower in volume despite being at the same level). The chinese versions on ebay you essentially get no support nor do you get any legitimate warranty.
Personally... I was totally fine with the prox blitzz units and was ready to purchase two more. However, I was noticing that a handful of the upscale venues I performed at were mentioning that they only allowed the sparkular units since they're tested and they are actually insured. No venues had an issue against the effect in general, and the prox blitzz would've been fine at around 75% of the venues I perform at, but because I aim to provide the best, I couldn't risk letting my clients down at the 25% of the venues that wanted a specific brand. That is why I made the investment into the sparkular units just months after buying the prox units. I knew I'd get my money back because I could already see the demand from the first 2-3 months of advertising it, and it's proven to be a profitable investment for sure.
As far as pricing goes... this will be market based. My area is more saturated and the companies using the cheap units have driven the price down. I list around $600 for 2 units and $1000 for 4... but that often is lowered by 10-25% depending on the package and if they saw it offered for less at a bridal show perhaps. Many companies are offering only 4 for $750 now, and it's an option I may consider doing. I say market drives the cost, as areas where fewer companies offer it, allow the few that do offer it to be able to charge more. In other areas I see dj's charging $500 per unit and sometimes more. I offer it for use during the first dance as well as during the dancing portions. Some companies charge less by only offering it for the first dance only since they're saving on the material use.
Let me know if you have any more specific questions on this effect.