I got my rails from Speedy Metals. The left one was perfect, but the right one (the more critical one) was junk. It is wavy, with about 1/8" variation one way and 1/16" the other. I've extended my X axis to 36", which makes it more of a problem.
Fortunately I waited to do my OnlineMetals order in case I messed anything up, so I ordered one from them and it was dead flat. Cost an extra $9 for my 36" rail, so probably $6 for the standard 24".