There's a list of the main agencies in Wales on the wiki:
External Link/Members OnlyBest bet is probably to have a look on there, see who you fancy and check for reviews on here.
I'm no agency expert, but I'd say on the whole Portfolio's girls seems to be generally well reviewed, but there have been instances of fake reviews.
If I was in your position, I'd probably be looking at trying to organise a couple of independents.