Those books are all going to be hard to find in print. You are basically looking for a used seller at this point.

Book 1 and 2 can be bought in digital form at Gumroad (and IIRC, have a bit of extra commentary beyond the print version, as the digital versions were published years later):
Book 1: https://richburlew.gumroad.com/l/ootsdcf
Book 2: https://richburlew.gumroad.com/l/ootsncfpb

Snip Snails and Dragon Tails has never had a digital version, and the print version is long out of print. Hopefully Rich will do a digital version at some point, but that's not the case now. So yeah, you'd just have to look for someone selling a used copy.