Depends on what you're looking for!
Most of the grocery stores carry Old El Paso brand & Discovery brand products. You may or may not like these.
You can find a few speciality items in the Food Hall - top floor of Harvey Nichols. I have seen some nice salsas and things like pozole kits there. Always worth having a look around Waitrose too.
Dried pinto beans (also black beans) are available in the shops, as well as tinned.
I tend to make my own Tex-Mex stuff from scratch, but I'll use the store-bought tortillas (like the Old El Paso or Discovery) & taco shells - to some that's heresy. Just all depends on your standards.
I don't like the corn tortillas sold here very much, so I'll probably just stick to the flour tortillas from hereon - if they're store-bought. I've been thinking about getting stuff to make my own corn tortillas, but haven't mustered up the motivation to go to the trouble (of kitting myself out with a tortilla press, masa harina & doing it) yet.
What are you looking for specifically?
Also, don't overlook online mail order sources:
http://www.lupepintos.com/http://www.coolchile.co.uk/http://www.mexgrocer.co.uk/http://www.southdevonchillifarm.co.uk/There may be others too.