Here is a short list of vendors/companies/sources of things I have used, purchased, etc.
I recommend them.
RF Parts (Many parts for home-brew linear amplifier and good source for valves)
The Amp Doctor (Based in Germany, they sell a good variety of parts. In some cases, the parts are cheaper (including the shipping) than the part from RFParts without the shipping: The new parts I ordered from The Amp Doctor were cheaper than used RF Parts INCLUDING THE SHIPPING FROM GERMANY. Makes you wonder!)
Mouser (Exceptional service and selection for any electronics) DigiKey is good but their web site is klunky compared to Mouser. Mostly the same parts, DigiKey might have a slightly deeper inventory for near-obsolete parts, but I much prefer Mouser.
Harbach (When I was in the middle of repairing Heathkit amplifiers, I used their kits. I would still recommend them.)
InRad (Good source for after-market filters)
Daily DX (Interesting reading. Mostly useful, but expensive!)
LZ1YE QSL Print (Very fine quality. Nice style. I will need to use them again for the next batch of cards.)