Go to your local Radio Shack and pick up a pack of crystals that they use for

there XMODS R C Cars. The set is 6 frequencies 7 thru 12 and work GREAT

in the Losi Mini T Cost is $ 15.99 for the set of 6

Be aware these are 1/2 step crystals as 7-12 are in between channels 1-6.
So, if you start having interference problems then you may want to look at your channel selection versus those running with you.

26.975 27.025 27.075 27.125 are some of these frequencies.

Standard 27 MHz frequencies are:
26.995 is channel 1 Brown
27.045 is channel 2 Red
27.095 is channel 3 Orange
27.145 is channel 4 Yellow
27.195 is channel 5 Green
27.255 is channel 6 Blue

They should work fine in most circumstances...the price is certainly good since typically you pay $10 or more for a single crystal set (TX and RX).

Also be careful to check, some places do not want you to use 27 split crystals.