12AX7 replacement

I would call Andy at Vintage Tube Services. You will probably need to tell him the brand and model of the preamp to maximize the value of his advice.

I too would recommend Andy, assuming you are going the NOS route.  He will not only know your unit, but can make recommendations based on the sound you like.  Tubes that sound good in some manufacturers' products will not necessarily sound the same in another's, that's part of the reason Andy is so good.  Another reason is that anything you buy from him, though pricey, will be the real deal and will have been thoroughly tested. 

If you want to go with current tubes and not the pricey stuff, if you still own that ARC SP3 and that's what you're talking about, I'd suggest asking ARC.

Brent Jessee at Brent Jessee Recording will give you good recommendations and has a large stock and will offer an optional warranty on your tubes (not a bad idea with NOS tubes).