Hi,
Sorry about the late response. Are you looking for left hand spiral end mills? In fact the full description is right hand cutting, left hand spiral. Down-cut is not a common description.

A left hand spiral would be used to force swarf downwards, usually in profile milling, and particularly if you needed to force a burr onto the lower edge where plastics are concerned. Centre cutting isn't an option.

As for where to find some, at an economic price, I haven't found a cheap or HSS option in the UK yet - sorry but my search skills are poor too. Uk sources may be limited to bigger manufacturers -we used them at work - in Carbide.
Is this what you need?
http://cnc-plus.de/en/End-Mills-266/...mill-2977.html

Good luck,
D