If you really want that Mesa scorching gain with feedback and all, you're gonna have to cook those tubes with a pedal in front of the amp. Just putting a high gain 12AX7 in V2 will not do it. I've found though that having a pedal in front of the amp with a high gain gives me more of that Mesa tone I love than just a pedal in front of the amp.
I've had a lot of success ordering from The Tube Depot, The Tube Store, and Doug's tubes. All three places give you the option of ordering a high gain 12AX7 for most of the 12AX7's they sell.
I've got a high gain Genalex Gold Lion in V2, and it sounds great. But it should at $45 a pop!
The only other high gain brand I've tried has been a JJ 12AX7, and I didn't care for it.