Try any of the usual suspects (tubeworld, thetubestore, tubes-r-us, etc.) for 6L6s.
The big power tubes mainly amplify the signal coming from the preamp. They sort of set the general "attitude" of the amp- Fenders used 6L6 or 6V6 a lot, Vox used the EL84 a lot.
The "little tubes" are mainly in the preamp section where a lot of the tone shaping of the amp happens.
Quick overview here from MesaBoogie: