I have found these websites to be really good;

fullermusic.co.uk (every piece of string music you could think of)
musicroom.com (a wider range of music, but not as much string music as above)

and now a plug for a personal friend:

0151 707 9123, 6 South John St, off Lord Street, Liverpool city centre, near Paradise St Bus station
there is a website at music-notes.net, although it doesn't have a stock list. However if you ring and speak to Lawrence, he is very good at finding what you want and posting it on in a few days. If you live in Liverpool drop in and nose round.