When I can't find a good deal and need a few real quick I will go to Harbor Freight. Made in China and around $16 ea, sometimes on sale for $12. They seem pretty decent though...I should give them a watertight test as I'm thinking about it.
I found some .30 cal cans the other day, the place is called "Prospector". 841 3rd Ave. in Longview, WA ($6.99 each). Yup, I have NO financial interest in them or any other company mentioned in this post.
As for 50 cal cans......I used to find them at the surplus place in Boring OR. Or maybe it wasn't actually......Boring, OR but it was close?
But, I did manage to buy a case of 50 cal ammo cans from Palmetto State Arms a while back.