A stripper clip or charger clip is used only for loading the magazine and is not necessary for the firearm to function, unlike an en bloc clip, which requires the clip with its ammunition to be inserted into the weapon to function. Generally, a weapon that can use a stripper clip for loading can also be loaded one round at a time, while a weapon designed for an en bloc clip cannot be used without the clip.

Which stripper clip should I use?

There's less than a dozen good, time tested reliable stripper clips on the market right now, and all are military surplus. The military can't afford to make a clip that doesn't work in a soldiers SKS carbine, vz. 58  or AK 74 at the right moment.

7.62x39MM BXN Czech, Yugoslavian, Russian, Old Circle 31 Stripper Clips

From all my research and hands on experience, I would rate them like this:
It is called a 'stripper' clip because, after the bolt is opened and the stripper clip is placed in position and  the user presses on the cartridges from above, sliding them down and off the clip, thereby 'stripping' them off the stripper clip and into the magazine. After the magazine is loaded, the stripper clip is removed and set aside for reloading, or simply discarded during combat.
Czech BXN clips work perfectly and are pretty cheap. Technically they were designed for the Vz.58 but they have worked 100% reliably in my 3 SKSs and in my friends SKSs. They're $4/10 from this guy, and he's also who I ordered mine from.. Only real downside is that he's shipping from the Czech republic so shipping is a little high, but if you order a bunch of clips it's enough to justify the shipping cost.
Alternatively you can go with surplus ammunition, but most of that stuff is corrosive, so it's up to you if you want to shoot it. If you have no plans to shoot corrosive ammunition and just want the clips, don't go that route.
The Circle31 clips work if you find the old grey colored ones, as those were made before their machines went out of spec. The new, black Circle31/NCStar ones require work to use properly.

Here's what a guy said on a forum awhile back, about stripper clips in general
Sell us your old stripper clips!
1.BXN Czechoslovakian Stripper Clips
2. Russian stripper clips- stamped 3Tk and very reliable-well made
3. Yugoslavian- Top rated also-not quite as smooth loading as the BXN..
4. Old Chinese 31. Phosphated. Grayish with a dark stripe down the middle.
and Circle 31. (Not the new Circle 31! They won't hold 10 rounds and maybe 50% even fit in the bolt carrier slot. Most won't fit the vz. 58 at all.) Ncstar makes these and ebay is covered in resellers that sell tons of these for 12.00-13.00 for 20 pieces. Not worth throwing your money away. You will be looking again soon, for better clips. Yes, I tried them myself.
Military Stripper Clip I.D. Chart 7.62x39mm
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Stripper Clip ID Chart