Where to find boxes for safely shipping Vinyl LPs?

Hi,

Are there cardboard boxes specifically made for safe and convenient shipping of just one or two LPs? Where can I buy them?

I have been trading LPs with friends and have found that it was hard to find the right box (thin, flat and strong) to ship just one or two LPs. I ended up using big boxes with lots of packing material, which raised the shipping cost unreasonably.

Any suggestions?

THANKS