Some are very easy to modify. For example this receiver at DealExtreme
is an easy one.
More information for adding a PPM pin can be found in this picture (see yellow wire):
I did the modification (on the 35MHz variant). I took a 3 pin male header. Removed 2 of the pins and glued the plastic part of the header with CA to the black box on the left side of the receiver (with the text CFWM - 455HT on it).
Europeans (35MHz) can use this receiver