Do these exist? Im interested in running RE2 on my recently bought JAP 64, but finding it doubtful I'll have much success on a regular N64 controller. Any input is appreciated!

Not that I know of. But the common alternative to shitty N64 controllers is to get a GameCube to N64 adapter.

http://www.raphnet-tech.com/products/gc_to_n64_adapter_v3_with_builtin_controller_pak/index.php

I was specifically wondering about PS1 to N64 bc it's for RE2, which I've only ever played using a PS1/2 Controller, and I've heard the Dpad SUCKS for RE2 on N64 haha. And that the GCN controller isn't much better...possibly worse lol

ugh.......... why would you even want to use non standard controllers meant for other systems in the first place. why not just get a controller that is multi-functional if they exist that you like? Use N64 on N64, Use PS1 and PS2 on PS1 or PS2. etc, that is what they were designed for and support.

Well they literally just said why - sometimes older controllers have issues that can be resolved with another controller.

For example, N64 control sticks are known for being hot garbage. Over time they wear out very quickly and end up not functioning correctly. So, people tend to use GC controllers if they can because they have much more reliable control sticks.

It looks like in the case of RE2, it needs a good control pad and control pads on Nintendo controllers can often end up sticking (at least in my experience) while PS/PS2 ones are better.