29" and 700c are two names for the same size wheels.

If you're putting them on a cross bike the limiting factor is probably not going to be available tire sizes, but frame clearance. Take a look at how wide the spaces where the tires will be are on your frame (between the chain stays by the bottom bracket, and between the seat stays near the brake bridge). Measure the space, subtract some 5-10mm, and that's around the biggest size tire you'll be able to fit.

Chris