Buying sight unseen from ebay could either win you a bargain or just some useless dirty felt. Best to try to buy locally or from somewhere such as Woolfest where there are loads of fleeces for sale from a wide variety of breeds and producers.
You could also look on the Shetland Sheep Society website to find a supplier near you, or look at the British Coloured Sheep Breeders Association website (assuming it's still on the go). In fact many of the breed societies probably have contacts for fleece sales.
There are also plenty of sheepkeepers here on TAS who are coming up towards shearing. If you can hang off for a few weeks you will get this year's clip which, if your project involves spinning or felting, is well worth waiting for.
As to price - well, all fleeces are not equal
. You could spend anywhere from £5 to £25. You could go higher but not for starters, and anything under £5 wouldn't be worth buying. Some fleeces struggle to weigh a kg whereas others are several times that, so worth choosing the breed you want before you go looking.