I got this one a while back. It adds the L2 and R2 triggers. A bit pricey, but comfortable enough to play.

L2 R2 Trigger Hand Grip Shell Controller Protective Case for Sony Playstation PS Vita 1000 (Black) https://www.amazon.com/dp/B0895YPHV7/ref=cm_sw_r_apan_glt_fabc_PZKRRRGV390K8SPKBJX6?_encoding=UTF8&psc=1