Ikea would definitely be the first place to check out.
Good prices and good selection. And definitely worth a trip where you can also get some of their other nice products (I especially love their kitchen wares).
You can also find good deals in their as-is section. Last time, I had found 50x70cm frames for only 2$ each... the deal of the century.
They have frames with either glass or acrylic in front. Many people I know don't like acrylic, something I can't understand. It is acid free, shatter resistant and blocks over 90% of all U.V. radiation. When you put a print under acrylic, the pigments resist much better to ambient light. I have inkjet prints I made over 10 years ago (back when inks were considered not good) that are framed in acrylic and they show very little signs of change... a very good track record for me.
Depending if your Polaroids are the peel apart type or SX-70, there are different options available.
For standard peel apart, a nice wide mat board in a neutral color could be nice. I particularly like offset mats where the image is closer to the top instead of the dead center type. It gives a nice effect but you'll have to cut the mat yourself as framing shops often don't do it.
For SX-70 type film, I feel the thickness of the print can be an important aspect of the image. Putting the picture on top of a nice board can be a good solution. There is also something I saw in a museum which I thought was very nice. They had the entire Polaroid blown-up and printed on photo paper. It gives the effect of an oversized SX-70 and is quite beautiful. The prints I saw were at least 20 inches wide...
