Do a search on "quick detach sling swivel" on sites like Amazon, Midway, or Brownell's, and you'll find a few styles and a huge range in price.

I see no reason to pay more than about $12 for these simple devices. As long as they will fit the sling you want, and are strong enough. Typically they have a 1" or 1.25" loop. You just want the plain plug-in unit, without any of the extra attachment parts.

Strangely, the only local shop near me to have one was Dick's Sporting Goods. They are quite literally THE LAST store I check when I want something "now" due to their idiocy over MSRs.