forever21 has cheap ones from 3.80 and up in stores and i think they have them online too.

They are not extremely budget friendly, but camis from Soma Intimates are awesome for the curvy girls.  They have elastic grips at the bottom that keep everything in place - NO MATTER WHAT.  They are a godsend.

Jessica London (www.jessicalondon.com) and probably Chadwicks.com have camis and tanks in plus sizes.  Their quality is usually pretty good.

The camis at J.Jill are fabulous.  They have a wide strap cami (way more comfortable than the lingerie straps when layering) with a shelf bra—they launder and wear well, too.

I have a hard time finding camis that come up high enough.  I mean, I don’t need a turtleneck, but if I’m layering for work, I really don’t want to be flashing cleavage (such as I have. :P) Also, does anyone else hate how so many camis have that useless bra in them?  It doesn’t support, and it tends to give me uni-boob.  So if anyone knows of a source of higher-necked, non-shelf bra camis, please share!

Try Old Navy - lots of styles, sizes & colors!

I also find the shelf bra a bit annoying; I found them perfectly acceptable as a less-endowed teenager, but since I reached DDs, they don’t do a thing for me.  I still wear them, just with a regular bra underneath for actual support.

I’ve noticed Chadwick’s camis tend to have a slightly higher neckline than others.

For camis, especially ones with a higher neckline, or for women who need actual support for their bust, try Norm Thompson. They have an underwire built in. They go from 32B-42DD in most cases.