My son has just decided Ben 10 is the best programme ever. I have been looking for tops and t-shirts for him, and was a bit put off by the high prices being charged on EBay - very lucky for me!.
The following t-shirts are currently being offered on EBay for £9.99 each, they are available on the high street for only £5-£6.
The two shown below are on sale in Tesco for £5 each. There is also a third design available.
These two are available from BHS for I think £6 each. They also have additional designs available.
I have also found Ben 10 t-shirts available in Woolworths for prices similar to above. Don't rush to buy, if you have the opportunity shop around!
As an aside, I found the Ben 10 lunch bag and sports drink cup in Asda.