As a rule of thumb you can usually order OEM software from the likes of eBuyer and dabs as long as you buy some hardware at the same time. I don't think it matters what hardware you buy - a floppy drive will do.

I suppose you can be counted as an OEM if you built your own computer, as then you are the original manufacturer of the computer. About the only downside to OEM hardware is it tends to be just the bit of kit you ordered and no fancy packaging or installation guides. If you're lucky you'll get a driver disc.