I buy my Scott mounts at the local stamp store here in Renton, Washington. I'm a long-time customer & they give me a 20% discount on mounts. I am a strong believer in supporting local businesses.
For stamps or supplies he does not have, or cannot easily obtain, I usually browse
ebay. If you are shopping
ebay, you need to know your material and read the small print carefully. But, I can honestly say I have had maybe 1 bad transaction out of 300-400, so that's not bad odds. I also have shopped on Delcampe.com, but seldom fine reasonably priced material there.
I collect pre-1950 mostly mint worldwide airmail. I design & print all of my own album pages, so that means always searching to buy philatelic reference material as well.