As far as I know, Geissele 6ARC mags are unobatnium. Every time I've looked on their website, they've been out of stock. Plus, they're really proud of these - so far - non-existent mags. Depending on capacity, they run from $70 (8 rnds) to $110 (25 rnds). 
The trick is growing up without growing old. -- Casey Stengal