Most suppliers that sell Linear Bearings sell Rack & Pinion for CNC so if you lookup "Linear Bearings" in the Yellow Pages you should find someone fairly close to you.
I find the guys a "Linear Technics" in Dandenong Vic., very helpfull and obliging.
Cost varies depending on where the rack was manufactured i.e. China.
15mm x 15mm is not too small but generally 19mm x 19mm is about right depending on the size you need for your bed length.
2 types of rack & pinion are common, "spur gear" and "helcal gear" both are good but helical is much quieter because there is always more than one tooth engaged, but it's also more expensive and the pinion teeth have to be hardened to reduce wear.
It's cost for a good China made rack is around AU$100 a meter, also it only comes in 2 M lengths so longer than that needs to be joined although the supplier will generaly supply a short length of rack to align the teeth with.
Hope this helps