Setting up your own DDNS service isn't particularly difficult; the hardest part would be to make sure the clients can actually do the updates (as I'm guessing these devices you have don't support the proper dynamic DNS update protocol, but instead one of the many ad-hoc protocols the DDNS service vendors have dreamed up). Also, if your devices are all network-localised together, running your own DNS infrastructure will give better performance than using an outside provider (who won't be as close, network-topologically).
If you need it done in a hurry, find someone who knows DNS service decently and give them a small packet of cash to set it up and write docs on how to maintain it. If it's no hurry, get a copy of the O'Reilly BIND book and have at it. If you can't get it done in a week of playing around, revert to plan A.
As far as using an existing DDNS provider, you'll automatically be reduced to using those which are compatible with the mechanism(s) available in your devices; beyond that, since you're dealing with large quantities of users they'll want money, and given that it's a commercial service you're running, you'll want service guarantees (more money). It'll probably be a custom arrangement you're on, so get your negotiating shoes on. My guess is that it'll probably cost as much in upfront time/money to get an agreement nailed down as it would to set it up yourself, and probably cost as much for ongoing service as it would to just run and maintain the setup yourself.
