www.militarygunsupply.com has PMKMS underfolders for $599.00
I'll actually get to see a few of these at the gun show this weekend, so I'll report back and let you know how they look. If you have an FFL in your area that will do a transfer for you, then you can have one shipped in.
As far as maintaining them, they're just like any other AK. There are manuals for field stripping an AK all over the net. It's probably the easiest thing in the world. Pull off the dust cover, take out the recoil spring and the bolt carrier, bolt and piston pull straight out. You should be able to find a manual at an Army surplus joint or a gun show.
The difference between Polish models is (as far as I've been able to gather) that the Tantal is a 5.45 side folder, and the PMKMS is a 7.62x39 underfolder. Mags for both are readily available, but ammo for 7.62x39 is easier to find (not that 5.45 is hard to find, you just can't walk into any sporting goods store and buy it).
AK accuracy with Iron sights is sufficient for hitting a 9"x9" target at 100 yards, but not much more. I've never used a scope on an AK yet, but I'm planning on picking one up tomorrow. I've seen some pretty impressive results that people are able to get with scopes on their AK's. Get a realy Ruskie scope or a real Ruskie red dot (like a Kobra). You can get a 8x42 scope from militarygunsupply.com for about $150, and there is another Ruskie selling Kobras for about $185.
I love AKs now. I have 4 variants and I'm hooked. I like 7.62's the best, because they have the authentic AK sound. These are more addictive than ARs, because you can get good AK's for 1/3 the price and really good AK's for half the price.
Good luck!