If you intend to do more that just a handful of computers, you should definitely look to get a trade supplier, as they will be able to get you reseller prices (which should be under the retail prices available to the public), and should also be able to give you good discounts as you reach higher quantities.

Unfortunately I've never been in that situation myself, and no longer know anyone else in that situation either.

I have vague a recollection of Misco doing it (as well as at least one of the ones I mentioned before), but as they also serve the general public they are unlikely to be the best.