I have had bad luck with RCA for some reason. They have a high chance of being microphonic. Not happy!!
Another great tube to score if you want to stay with 12AT7 would be Telefunken. That is the best 12AT7 made! Ei makes a clone of the Tele tube that is extremely close in tolerances and sound for about 12 bones!
Oh, and I guarentee that the tube that came in your mic was probably not a 12AT7, but rather a 12AX7, so practically any tube replacement (12AY7 or 12AT7) will improve the sound. the lower gain of either of these tubes makes it harder to saturate the transformer. You will much more headroom and transients will be better!