Blain, you can get the "Duckbill" SKS mags all over.
CDNN has them as does
Cheaper than Dirt.
However, reliability is spotty with them. I've used some of the USA brand and some Chinese, I even have two chinese 20 round fixed mags. I could never get any of them to have any acceptable reliability for any length of time.
There are also legal issues. According to the laws of importation(9.22r), semi-auto firearms with detachable "high capacity" magazines are severely restricted. Basically, by adding a detachable hi-cap to a Yugo SKS you are creating an "unimportable Assault Weapon" because it has a bayonette mount, grenade launcher, and night sights.
Believe me, I've tried to get detachables to work with the SKS and even when to the extent of removing all the other "evil features" on one of my shooter Yugos to try and keep it legal. It's just not worth all the work and aggrevation. I ended up selling all my duckbill mags, putting back the original fixed mag, bought a 100 pack of strippers and a couple Chinese bandoliers.
SKSes are extremely reliable rifles with the 10 round fixed mags. I've got 7 of various variations and I can't remember one failure with the fixed mags. My advice is to learn how to get good with loading strippers and enjoy it for what it is. Then save up $300 and buy a SAR1 at a local gun store/gun show to enjoy $7 30rd, $14 40rd mags and $90 75 round drums.