Wifi is much easier to find - airport express would be the obvious cheapest option at £65, which could then be plugged into the digital input of the receiver. There are others such as Squeezebox or things like the Linksys Media Extender or Apple TV which also do it, though their additional functionality comes with additional expense - just depends whether you want a visual display or not, and whether you can plug it into the telly
However, if you don't already have the receiver, some are network-enabled themselves (Onkyo spring to mind), so you could go down that route.