I got some stainless hi-caps from Sportsman's Guide for around $11 each, after Buyer's Club discount.
(I think they were Promag, but the people at S.G. couldn't tell me.)
These will work if you are willing to "tweak" on them.
Some had to have the mag body compressed a little, so they would fit and drop free.
Some had to have the lips bent in a little, so the slide wouldn't lock with the last round still in the mag.
I replaced the baseplate, spring, and follower with Ruger parts.
They work like factory.
If you want real factory mags, which are the best, try the following:
www.ebay.com
www.gunbroker.com
www.auctionarms.com
These are all auction sites.
I have found factory Ruger hicaps for $28 each delivered.