One place to get individual cogs is
SJS Cycles. Their delivery times are often shorter than from US vendors. The general truth, however, is that the cheapest way of getting individual cogs is to buy a whole cassette...
Are the cogs a standard thickness between manufacturers?
For that matter, how about using cogs off a much cheaper 9 (or even 8) speed cassette with 10 speed spacers? Or are the cogs thicker or otherwise different? I understand the ramping isn't likely to match.