Enjoy your new addition. The 22/45 is a great gun. I just shot mine today for the umpteenth time, and I'm still impressed with the accuracy and reliablility.
http://www.cheaperthandirt.com
Cheaper Than Dirt 1-800-421-8047 has them on page 10 of their newest catalog. Factory Ruger 22/45 Magazines, 10 round, MAG-439, $13.69 each.!
Has the best price I can find on MKII,22/45 and 10/22 mags,If you get 5 they nock off a buck per mag,the regular price is 12.99 for the 22/45,with shipping they run about 12.50 per mag if you buy 5. You can find them in shotgun news or http://www.KYIMPORTS.COM the web site does not list the buy five deal just call them.
They're around almost everywhere in the $13-15 range locally. Hell, I think you'll spend more effort and money trying to save a buck than just buying them when you see them.