Seeking to purchase Mac Pro Towers - Suggestions?

Hello All -

I am currently building out a server farm of old Mac Pro servers. (The bigger silver ones, prior to the new black ones.)

I am interested in purchasing as little as 1 and as many as a few hundred.

I was wondering if anyone had advice on how/where I can solicit some companies that have a lot of Mac Pro's that might want to upgrade to the new ones IF they knew they had a buyer for all of the old ones?

Thanks!