I've used these guys before (UK based) for their Belin Single Flute Downcut bits... not the cheapest but good quality.

http://www.cncroutershop.com/uk_en/

They have a good selection of Single Flute upcut bits too, including their own brand.

Other than that I tend to use AliExpress, most stuff comes within a couple of weeks at most even slow boating. I've used these guys before and quality was great.

http://s.aliexpress.com/QRnaE3ia

Roughing bits might be worth looking at too:

http://s.aliexpress.com/j6vqyuAN