Pooleys or AFE I would suggest as good places for purchasing one online.
It depends what your son is aiming to do, if it is just a PPL for recreational purposes then a CRP1 flight computer is plenty sufficient. About £40-£50 ?
If he is going on to do further flying, which I'd guess he is, then investing in a CRP5 or similar is worthwhile. About £70 for that.
Other types out there, but just used those as examples as they've what I have used.