Have a look at nRF24L01+ adapters. Really cheap on ebay. You can get ones with a tiny onboard antenna (99p) or slightly more expensive ones with a bigger screw-on antenna (~£3).
Edit : If you want to buy some on ebay the cheap ones come from China/HK. Also, since you want more than one search for something like "
nRF24L01 -1pcs pcs" to get some good prices for multiple adapters.EDITED: 3 Jan 2015 22:33 by ANT_THOMAS