How difficult is it to replace furnace ductwork for a ranch house with a crawl space? Where can I get supplies to replace my furnace ductwork?

Try Home Depot.

Right under your question is an ad, Ductwork For Homeowners Install All Your HVAC Ductwork Trunk, Flex, Boots
Audubonsupply.com

How deep is your crawlspace and what is the floor made of? Are the undersides of the joists open, or insulated with a vapor barrier? How old is the existing system? What climate are you in?

Why are you planning on replacing the ductwork? Sounds like an easy job that anybody could do, right? Wrong, there is a reason that there are tradesmen who do only ductwork. If you do decide to take this on yourself, just make sure to do your homework. Good luck on your project, and please let us know how it all works out.