There are a number of companies offering pre-recorded mini-dics. The best one to try first is Sony Music's site - they are pioneers of the technology. and logicaly will have a large selection. Click on www.minidisc.com. Another major merchandiser of recorded material on the net is Amazon, find them at amazon.co.uk and if you prefer an independent retailer, then a click on [email protected] should provide what you need.
I'm not sure if this is still true, but about a year ago, when everyone thought that minidisc would really take off, the really big branches of HMV (and, I think, Virgin Megastores) in London had a few shelves of Minidiscs. The range was pathetic, and they didn't seem very popular, though.