Many of the discount places have tubes for relatively cheap. Like Musician's Friend for instance.
I buy my tubes from Eurotubes. They sell the JJ tubes and do a very good job of trying to help match you up to a set of pre-amp and power-amp tubes that satisfy your desire for tone. I've emailed the owner several times and I've retubed my Traynor Custom Blue 50 with them. I bought the whole set of pre and power tubes from them for about $60 I think. I will be retubing my Traynor YCS100 with them shortly... as soon as I get out of this hole that my recent IRS payment put me in.