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Try a crafts store or hobby store for flat cork. It will be about 1/8 inch thick. That's what you need. Out here the stores are called Michaels and Hobby Lobby. It comes in small pieces, about 4" square or so.

The Box-o-Truth article (linked in the UptheIrons post above) is the source on corking. The article is well laid out and easy to follow. Corking is an inexpensive technique to improve accuracy on some mosin nagants, although it doesn't work equally well on all guns. Gotta love those MNs.

My experience is that the MNs get an undeserved reputation as being significantly less accurate than other milsurps (eg. the swedes and swiss guns). If you shoot good ammo thru a MN (eg. S&B) you will generally get better results. Even the best surplus 7.62x54R ammo is erratic at best. I've shot bulgarian, czech, and russian, and the results are spotty at best. Don't presume there's something wrong with the gun when it may just be the ammo. Besides, there are some inexpensive measures that can improve accuracy on a MN (eg. corking the end of the stock). I love my swedes, and I also love my MNs. just my 2 cents.