External Audio Filter advice?


Looking to add an external, tweakable, filter to my signal chain and would like any recommendations on how to get there. I’m happy to try to build something, or buy a ready made product. Basically have two digital synths and I want to run their STEREO signals though an analogue filter. It needs to be stereo and not powered by batteries. Filter and resonance would be great, an lfo would be rocket ships, but if it’s just a filter that is okay. What do you recommend? is there a place to just buy filters, or do you recommend a product? I bought a Korg monotron for this and it does a pretty good job, however, it seems there is not a way to a/c power it without adding noise to the signal (tried), other then using batteries, which it eats. Like to go low cost if I could. Anyway, thank you for your help!