If you have a router or table saw you xan make yourself some easy cheap doors , i just did it for the first time myself came out great , i woukd offeer to kend you the tools but i borrowed the router myself from a neighbor
Ive heard homedepot has doors as well you can order in any size not sure how much they are though , just for refernce it probably cost me about 40$ a door to build a very very plain door from the cheapest possible wood (poplar and birch plywood)
Cabinetsnow.com has some inexpensive options.....like $15-20 per door. Or baker. There all made the same way. Difference is when you have an assembly line making them they can do it cheaper then your average carpenter. Im sure some will argue quality control is lower but it's not like were building 10,000 sqft custom homes.