there is a good thread over at the Drawing Board. It is curently about 5 threads down.
Every transformer company specs the usual 20 to 20 khz response, but this is probably one of the most useless specs on a transformer you could have.
More important is distortion at low frequencies, resonant peaking, phase shift, leakage, capacitance and inductance, stuff you can't always find, so you just have to go with the reputable dealers that are still in bussiness, get the right transformer for the job, and hope for the best.
There are the usual dis-continued products that some people like, UTC, Peerless, Triad, which are still made by some small shops, but they cost a lot more than stuff you might get a deal on in an ebay auction.
Also, sometimes you want color, and not great specs. This is where some people like to go cheap on purpose to get a little tone.
cj