Hi Tom, when my partner and I started chastity we bought a cheap CB6000 clone off ebay for less than £20. Soon upgraded to a spiral clone which has served very well. Just got a Prince Albert and now live in a 2nd hand Tube custom that I got off the market place forum on here.
I'd suggest having a good look at ebay as you can get most devices and choose one that you like the look of. I'd also recommend getting one with several sizes of rings so you can work out what is comfy.
That's my 2p as a relative newcomer!
M