Most registrars get a bag from their surgery. A few do not and every registrar would need to buy one when they finish training.
There are many, many bags around. All one needs to do is go the website ofmedisave or wms or pottertons and you should find one that suits your need. They start from £80 and go upto £400+.
I have bought a bag recently and am posting it's review here. I bought if from the BMA medical supplies website and got it for £ 136 including VAT.
These are a few pictures of the bag.
First impressions: I like it. It is not very posh, made of polyester but is very light weight(1.5kg) and quite practical. 
Update (9/10/09) : It has survived 2 months of visits. No scratches as yet. My only gripe so far is that it is prone to falls when placed vertical
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