If you click on any of the posters on that website, it tells you who the publisher is. In the past we have been able to contact the publisher and buy directly from them.
I'm not sure what type of sports posters you all are looking for, but Photofile.com has a link to wholesale customer contacts.
Or, Puresportsart.com / sportsposterwarehouse.com had this on their FAQs:
I would like to carry some of the posters found at The Sports Poster Warehouse in my retail store. Do you offer wholesale pricing or volume discounts?
The Sports Poster Warehouse is not a wholesaler. We buy our posters from publishers and other wholesalers and sell them at retail prices. Therefore we do not have wholesale rates to offer. We do offer volume discounts on orders of 15 posters or more. If you would like to place an order of this size, please call our toll-free number (1-888-650-3455).