Down in OC you\'ve got GW\'s ubiquitous presence, with their store at The Block in Orange and their LA Battle Bunker location in Westminster, but you\'ll also want to look at
Brookhurst Hobbies in Garden Grove. It\'s been a while since I\'ve actually been in the store, but they\'ve always been fairly good for miniatures in the past. (And, this response has reminded me I have a small gift certificate for there from Christmas I need to use -- thanks!

) Those three are all in the realm of 10-18 miles from Irvine in general, to the north-northwest. The store finder on GW\'s site also lists a Comic Quest in Lake Forest, about 14 miles southeast of Irvine -- they could have other mini stuff, too, if they\'re carrying GW, but I have no idea and no personal experience at all.
Also, another possibility is to see if anything you want is in the eBay store of
daloonieshaman -- that\'s run out of the home of a guy I know in Pasadena who runs the Blood Bowl leagues I play in every now and again. If I happen to need something while a league is running (meaning I\'m playing in his garage every couple of weeks), I\'ll often just buy directly from him face-to-face. That could be a possibility for you, too, if you wanted to show up at a gaming night he holds every so often.
As for temporary positioning of minis, I use poster tack, works well enough for me: