Hi Moodykat. Great decision - bell tents are fab! There may be camping shops which do sell them but I think most people tend to buy them on the internet, often from this website: www.soulpad.co.uk which I have seen recommended for good customer service too. Ebay is another option. However, if you do a Google search lots of other sellers will pop up too.
The more expensive tents are often priced higher because they are made of thicker canvas, or are made in this country, or some similar reason. I have no doubt they are better quality but, having had two bell tents of the basic grade of material, I have never had any problems with them, and with higher-grade canvas comes more weight. Up to you of course.
In terms of where you might see them, some of the online sellers now do have showrooms and if you go to a campsite, particualrly of the sort which allow campfires, you are increasingly likely to spot a few. Most bell tent owners are very friendly and proud of their tents ime and will happily show you round.
The cheapest place to buy bell tents is from www.obelink.nl. However, they do not deliver to the UK (due to a restrictive arrangement they have with a certain UK seller), so you would have to take a trip to their shop in Holland or get them to courier to somewhere on the European mainland (we had an inner tent delivered to a campsite in France last year, but you do have to be careful with dates and delivery times and get the campsite on side if you are going to do this). You can save a lot of money this way though, particularly on the basic tent, without a sewn in groundsheet (SIG) or zipped in groundsheet (ZIG).
You could do worse than look at old threads on here about bell tents as there is lots of useful stuff about SIG v ZIG v PIG (pegged in groundsheet), whether to go for a 4m or a 5m (I have had both and I do think with a bell tent, the more space the better, particualrly if there are more than two of you).
It is also worth thinking about the type of person you are. The nature of a bell tent is that everything is all together in one large room. You can buy inner tents but a lot of people feel they ruin the whole open feeling which is one of the best things about bell tents. However, open plan tends to mean it is harder to be tidy in a bell tent than in one with lots of different pods and separate rooms for sleeping and living. If this bothers you, a bell tent may not be for you. On the other hand if you think prettying up your tent with bunting/cushions/throws etc will make you happy and you don't mind doing a bit of tidying each day, you can't beat a bell tent.
Whatever you decide - Happy Camping!