The 'shortage' has more to do with limited production than anything else. These are not standard capacity in most of the rifles shipped and few people want more than a handful of these so they are not pumping them out like the 25 rounders. Plus I think these are made by Magpul and they got quite a bit going on so these probably don't go into production too often.
Your best bet is to either watch these threads:
10 Short thread #1
10 Short thread #2
Or put a couple on backorder and wait:
MidwayUSA
Good Luck and don't hesitate to grab them if you see them in stock and under $20