The big thing in Meridian is neighborhood garage sales. Watch the Idaho Statesman (in print or online) for them to advertise and most also have a sign that they put up at least a week in advance.
The best times to hit the sales is now through mid-June when a lot of families go on vacation or it gets too hot for some people. You'll find a little of everything out there.
Also, boise.craiglist.org is another great place to find things cheap! Think of it as an online-garage sale. I know that we've listed a lot of stuff on there and have received fantastic responses. We've also found a lot of good, affordable stuff. It's a good way to shop if the weather is bad or if you're looking for something specific. Downside is that you don't get to browse through other stuff!