If your samples are small enough there are some very sweet LED darkfield illuminators made for microscopes which project a plane of light into the centre of a ring. They are not usually specced for the thickness of the light sheet, but I'm sure their application engineers can help you out. One example:

http://www.microscan.com/en-us/Produ...uminators.aspx

There are also LED-based precision line illuminators which probably project a thin enough line (or can be focussed). Three or more of these would be easier to align than a bunch of projectors, and they come in lengths up to half a meter or more.

Not cheap, but they're a) aligned, b) even and c) colour balance controlled. Homebrew solutions are possible, but aligning the teeny-weeny LED emitter to a collimator or line-projection optic isn't an easy job.

If you can live with B+W, a laser pointer with a clip-on line projector will work, and again, will be easier to set up than a projector. Green pointers with pattern generators are fairly commonly available - but watch your eyes if you start peering into focussed laser light, some pointers on eBay and elsewhere are not eyesafe if focussed.