Jac's version, "I only have a little left", is correct.
"only" makes "a little" sound negative (meaning that the person is short of cosmetics).
"a few" (not few) would also be possible, but this would suggest a variety of cosmetics, not cosmetics in general as is the case with "a little".
Curiously, "only" requires those pronouns to be preceded by "a", which can only be omitted if "only" is left out:
- I only have a little left.
I have little left.
I only have a few left.
- I have few left.
We can of course also say:
- I have a little left.
- I have a few left.
but these will not have the restrictive meaning that is intended.