Which online store have Bartolini MPXTBT onboard preamp

FWIW:

never heard of it but if it's a current version of the NTBT then any distributor having access to the NTBT should be able to order it. Nobody's likely to have a rare item in stock. Do an online google search for NTBT and email the distributor.

If it's a version of the old TBT, nobody will have it unless it's new old stock cause production has ceased.

Bart doesn't have email currently (last check) but you can call them at the number listed on their site.

 

I've bought Bartolini pickups from luthier Jack Read before. He is a great guy.

 

I'm not sure if they have that particular model, but these guys are top notch, and do excellent installs if you need it.

http://guitarelectronics.zoovy.com/