There are lots of online resources for DIY audio. Rather than making an exhaustive list, I've made a list of places where I have either bought stuff or learned stuff, or both.

Parts Suppliers DigiKey Mouser Newark Allied Electronics Parts Express Audiophile Parts Supplers Michael Percy Audio Sonic Craft Partsconnexion Hand Made Electronics Tubes, etc. Tube Datasheet Locator Frank's Electron Tube Datasheets Radio Electric Supply Antique Electronic Supply The Tube Center Tube Depot The Tube Store Transformers Electra-Print Lundahl CineMag Intact Audio Sowter James Magnequest Other Useful Information DIYAudio.com Pete Millett's DIY pages Pass Labs DIY Other Useful Stuff Online Metals Front Panel Express