If you cannot find the hairpiece you are after for your custom minifigure then create your own. This fantastic guide by tomleech goes into great detail and is definitely worth a read:

I sculpt in kneadatite and I like to get a good smooth coverage (without fingerprints) the right shape before I start adding detail. I find oil helps with this (olive oil, vegetable oil etc.)

I also have a few different tools:

The metal one I use for general shaping and cutting, the ‘angle chisel’ for doing sharp lines in hair (joker, two face, my own fig etc.) and the ‘taper point’ I use to poke the hair to get the dimples where a curl is.

This is pretty much how I do it, dimpling and adding depressions to join the dimples together, looking for partings/higher and lower areas etc.

I also created a tool from taking the impression of the back of this hair which I press all over the smooth hair to give me some wavy impressions to start with, although this pretty much just gives a bit of texture rather than making curly hair, as you imagine it should do (possibly because it’s such a small amount to work from and it repeats too quickly).