Found some for $53/shipped. It seems like a decent price for a 70+ round drum, then again I've never purchased any.
I picked up about 6 and got put on the Lage Max-31 list so I have a reason to use them.

They arrived and look to be in good shape, although one has a few dents/dings. But I won't be able to test them until the Max31 upper arrives someday. Are the drums generally reliable or should I look for the "coffin mags"? Is this a good price or no? Would you recommend I get more or is 6 going to last pretty much forever?

I have about 30 sten mags (including some 50rnd Lanchesters) that I use for both my OEM M11/9 upper and Max-11 upper and I don't think I've used/unwrapped more than half a dozen in a decade of use. So I only bought 6 drums... which is still almost half a case worth of 9mm loaded up (although in hindsight, I should have gotten 7 drums because that IS a half case ).

This guy had some for sale. Not sure if any left but worth a try.

http://www.uzitalk.com/forums/showthread.php?84290-Suomi-Drums-and-Parts-For-Sale!!!&highlight=Suomi+drums+for+sale