Aiken Amplification--Great Guitar AmplifiersRandall Aiken (Tube Guru) makes great amps and is a really nice guy to boot. Plenty of Tech info on his site as well.
Kendrick AmplifiersAnother "Guru" of tube amps like Hoffman and Aiken. Gerald Weber has his own amp line, Guitar line and some excellent books and tapes on tube amps.
New Sensor--Amp Parts and other things GuitarThe Sovtek and Electro Harmonix dealer of choice. Get tubes, caps, trannies and many more items here.
Ampwares--Amp Parts + the Fender Field GuidePretty much anything you need and a great resource on vintage fender amp data.