Despite what Anke says, pedigree Shetland prices are much the same as unregistered sheep. Show sheep can command slightly higher prices, but not a great difference. If you do want to eventually breed, then buying some of this year's lambs, August time, would mean you have 15 months before you need to think about breeding, plus some wethers for the freezer. There are plenty of breeders in your area. Going to the York rare breed spring sale or some local agricultural shows would allow you to make some contacts, and the Shetland Sheep Society web page has a page giving details of breeders with stock for sale by area.
By buying pedigree sheep, you are more likely to get sheep with good Shetland fleeces. The finest ones can be sold for £10 or more to hand spinners. White fleeces are of very high quality, but the point of Shetland sheep and the society in the first place was to preserve the coloured and patterned sheep. Fleeces from southern sheep are not shorter. Going for crimpier fleeces can make them seem shorter. The Shetland standard for fleece has a size range that most pedigree animals meet. The gene pool is quite large, I think.
Try and ensure you buy sheep that have been well handled and are bucket trained. They will be friendlier and you will find them easier to handle. Half a dozen would be a good starter flock, that you could build up by breeding over the next few years. Had you wanted to breed this year, I would have suggested buying some older ewes who have bred before, and know the score, which would be easier for someone lambing for the first time. Hope it goes well for you.