Thanks Chegs. As it happens, I'd already done a Google search and couldn't find anywhere that sold 256MB cards, which is why I asked in here. I thought that someone might be able to tell me for sure if they existed or not, and if so, provide a site which I had obviously overlooked.
Smart Media only have capacities upto 128mb, if you require greater capacity you will have to look at alternative cards, subject to your equipment supporting them. For example Compact Flash have attained capacities of 1024mb.