As wantmygti states, after checking the exact size, including the load/speed index, log onto mytyres or blackcircles - get a ball-park figure.
Then look in yellow pages, and phone around a few, shopping around for prices. On my S4 boots, I managed to get nigh-on £80 per corner knocked off some prices - now that is worth half a day on the blower!
Avoid Thik-Fit at all costs, and give these a try
http://www.protyrefareham.co.uk/ - they have a nationwide reputation as being truely excellent - and pretty close to Phil too!