Grammar: While stocks last; while stock lasts

While stock lasts - is correct.

This is because stock is uncountable and hence we cannot put an s after stock.

2008-04-16 01:14:26 補充：
While stock last - is wrong.

This is because after a singular noun, we should use third person singular verb, that is, an s has to be put after the verb.

While stock lasts is correct.
Since stock is a collective noun without plural form, thus a singular noun should use s after that.