Jill’s outfit is from Club Crimsyn and can be found under TSR Sets / TF. I download a lot of clothes for my teens (and adults) from there, so I’d definitely recommend that site.
I’m not really good at remembering where I have downloaded most of my other teen clothes from, but a few blogs that come to my mind:
Rented Space has a lot of very nice clothes, and some of them work for teens.
I’d also generally recommend going through GOS, MTS, Sims 2 Defaults (even if you don’t want them as defaults, there are mostly links to the custom versions of the clothes) TBTO, Trapping Crushes and Sims Cave’s updates.
Hope that helps! :)