Jeff's Outfitters has some nice ones. I find his prices are less than the big name guys for the same product and you can order on the web.

I have the canvas one on my Heym Sfari and on my Simson Suhl 12 gauge. They are well made.

Here's the link to the slings:

http://www.jeffsoutfitters.com/store.aspx?categoryid=62&panel=1

I think Galazans (Connnecticut Shotgun) also carries them.