Anything from Digikey or Mouser are top notch. I usually calculate the price per square inch when comparing. RS is crap for sure, but you can work around the warp with double sided tape covering the whole backside of the board to pull it flat. I also have a stock of PCB I bought from MPLS.com.
I have found I don't need 1oz copper or 1/16" boards. So I like the 1/32" PCBs that Pulsar Pro FX sells.