try a specialist blind company, who will come round and measure up.. but don't use Hillarys blinds (if you have one near you) they are a bunch of miserable so and so's who are unhelpful to say the least...
Try www.247blinds.co.uk my daughter bought blinds for her conservatory from this site, excellent service. They will send you a sample of the material, she received hers the next day, very quick after ordering, and far cheaper than the high street shops. There are other sites that also do blinds.