TCS conferences for undergraduate students?
The POPL-colocated PLMW, which you mentioned, certainly provides travel support for undergraduate as well as graduate students.  I personally know undergrads who've been supported. If you're interested in PL research (of any kind, not just POPL-like research) I encourage you to apply.

The ACM student research contest (of which the ICFP SRC is a part) includes categories for both undergraduates and graduates. For SRC at SIGPLAN affiliated conferences like ICFP or PLDI, you can apply to the SIGPLAN PAC program for travel support.