Just bought a few cutters yesterday from these people - http://cutweltools.co.uk/milling_cutters.aspx

Ignore the price's on the internet, give them a call and tell them that someone recommended them and you would like to try a selection. I got them for £2.50 each - K2 coated solid carbide throwaway's - 3mm,4mm,5mm,6mm and 3mm ball nose, short and long versions of each and all with 6mm shanks.

A friend of mine uses them all the time in his cnc machine shop for milling steel and cast aluminium and says good things about them.

Just noticed that they do special cutters for aluminium, I might give those a try too.