best bet would be your Yugoslavian M70 underfolders. The problem however is that they have since dried up. Last year around this time, they were everywhere and would cost only 400 bucks. Then, by mid-Spring, they were 550. Now, your best bet is to browse gunbroker and bite the bullet on whatever outrageous price you find one for.
Finding your GP-WASR underfolders is now a hassle as well, but those shouldnt be more than 500. I see a fair deal of them at gun shows, but they are not nearly as nice as the Yugos -- not even close.
Honestly, finding an underfolder worth the money is now a thing of the past in my opinion.