Holt Model Railways is your best bet, although their web site doesn't appear to be working at the moment(useful for checking what they stock) . Voyles on the Trostre estate is good , don't be misled by the fact it also seems to be a TV shop, theres plenty of stock but he doesn't have a website. R&R in the City Centre(an arcade) is only open Friday and Saturday for Model Railways(which are upstairs) and then limited hours. A Trip to Cardiff will give you Lord & Butler and they have a really good shop plus Antics in the City Centre.