Hi mate,
Yep big difference between the two (as I found out after burning out two N/A ones after my TT conversion!) Erm, I cant remember the part no off the top of my head, I'll have a look tonight, but I'm sure someone on here will beat me to it! The N/A alt is around 80A and the TT is 90-100A IIRC (TT one is bigger and heavier basically.
Second hand or refurbed ones ones are as rare as rocking horse poo as far as I'm aware, I was in a rush for mine and had to fork out £395 from Steve Manley (thats including the £35 rebate I got for sending him the old one back!!) and it was the last one in the country apparantly!!
Good luck with finding one, the two auto sparky places I went to around here looket as iif I'd swore ate their mums when I asked whether they could get hold of one!