Zach:
I’d buy my slide film from B&H. Velvia is about $5.50 a roll and Provia $3.50.
I’d usually buy 20-40 rolls to get the bulk price and to save on shipping. Store it in the frig or the freezer. I’ve used their A&I film mailers and the service and quality is good. It just takes a while from California.
The Slide Printer in Denver also has good service and they used to pay postage. You may have a quality mail lab on the east coast there.
Only ever lost one roll of film and apparently it was the post office’s fault. But it had the best sequence on it of a heron swallowing a fish I had ever taken. :'(