I play Oblivion Game of the Year Edition on the PC (so I have Shivering Isles and Knights).

I had previously purchased via an online Bethesda store (and a now dead HD) copies of the Frostcrag, Spell Tomes, Thieves Den, etc.

I have found this via some arcane weblinkage ->

http://www.elderscrolls.com/oblivion/dlc/

But this only seems to be for xBox.

Are these DLC no longer available for the PC (short of a Steam insstall, which I am not doing to do at this time)?

I seem to recall they were quite cheap, on the order of $2 apiece when I bought them previously, so I wouldn't expect a single purchase to count for multiple downloads and I'd be happy to pay for them again.