I've purchased a lot of promotional supplies (t-shirts for Startup Weekend, pens, business cards) from a company called AutoMall Direct. They mainly provide items for auto dealerships but they accept orders from anyone and are usually 10% below others' prices. No website (keeping their costs down I guess!) but their phone # is 800-228-2556.
(They'll send you links to the suppliers' website to look at specific pen models, of course.) Most promotional pen companies all use the same wholesalers. The main factors for you to consider are (a) price and (b) whether they can steer you towards quality pens or not. Ever had a pen run out of ink right when you needed it? You don't want your name on that pen.