You've got lits of options when it comes to Skoolies.
I'm still searching for the right bus myself but what I can tell you is that it's not that diffcult over all to fibd a good deal.
The vast majority of fleet companies dont like buying used buses of any variety so resale is usually low. On thw bring side, after your conversion of a used vehcile you can usuallu recoup most if not all of you investment when you decide to upgrade (so long as you dont have horrible interior decorating skills and a passion for 80's wallpaper).
Anyhow, you can search for school buses or you can search for Shuttle buses. Both commonly have lifts.
I dont know where you live but here's a school bus sample in new york state.
Also, if you rnd up finding skoolies in your area, many have little use for the lifts and while they cost thousands new, there's no market for ones.
Hope this helps get you started.