Welcome!
The number of truck conversions on the market is hardly a measurable fraction of the total number of motorhomes out there, but there are still quite a few nice ones if you know where to look.
I worked with two very helpful dealers: RacingRVs.com & DixieAutoBrokers.com. I think they were recommended to me on this forum. I purchased my truck from Racing RVs but both dealers worked very hard to find what I was looking for.
It is also a good idea to contact any dealers near you that sell your preferred brand and let them know what you are looking for. They could have a trade in on the way, or know of a current owner thinking about upgrading who may have what you want.
And as mentioned above, racing junk is a great place to shop as well. Good luck with your purchase!