Has anyone found the xbee "chip" (any of them, Series 1 preferably) separately sold, not broken out on a usual xbee module (so that one could solder it directly to their own custom pcb)? I tried mouser, and they have some zigbee chips, but all of them are of the "mesh" series and seem to be unrelated to the xbee.
Yes. They are available. I have bought 4-6 of these locally... you can try digi-key or farnell.