That appears to only have cans of wet beans, which are a lot more expensive than bags of dry beans. This site has 50lb bags of pinto beans, but I was looking for something local to avoid shipping costs: http://www.aaoobfoods.com
I think you're making assumptions that may not be true.
Note the page specifically says that they sell dried foods, etc.
I have some cans of dried beans myself, and they are a little over 5 lbs, as the ones on that webpage are also. So, for that size can, 5 lbs of beans would imply that they're dry. Wet beans would probably weigh more. (sorry it took me so long to post this as a reply, but I had to get around to checking my cans so I could genuinely confirm this for you, rather than just speculate about it.)
Anyways, I posted that site as only one example of many sites/suppliers where such things are available from. There are most likely a number of more local sources.
I don't know how often or in what volume you eat beans, but I think getting dried beans in cans rather than bags is probably a much better idea when you're talking about anymore than having a few pounds around. Since you said you wanted to buy 50 lbs, you're going to have to come up with some way of storing it yourself that is pest-proof. Prepackaged cans can of course sit in your storage pantry, unmolested, for umpteen years without any need to worry about them.