I am envious of that. It's not hard to get pony shirts here, but we generally have to order them -- and in most cases from the US. There are occasional sightings of a Derpy shirt in a UK Forbidden Planet (chain of comic-etc shops) but that's about it. I was interested when checking the shirts available on the FP site, though to find that they're not all G4 (http://forbiddenplanet.com/clothing/?q=My%20Little%20Pony).