Trying to volunteer as a research assistant post-Master's and not having any success so far I'm an MS in CompSci grad who wants to do a PhD someday. The one thing I don't have from my time in undergrad and MS is significant research experience. Of course, I did undergrad research but was never published in any papers. I make a lot as a professional and want to volunteer(read, free) part-time doing research for a professor or lab post-graduation, but I have no idea how to approach this or even find interest amongst professors or labs. Any ideas?

Edit: Undiagnosed disability crippled my undergraduate performance. I was unmedicated, untreated, and had no idea on how to cope. I really managed to redeem myself through my graduate performance and am basically looking for a second chance. If anyone has disability-specific resources as well, that would be great.
It's probably going to be a bit difficult to get a professor to agree to take you on as a "volunteer," because they see it (in my personal experience) as a time-sink from what they're really trying to accomplish, which is train students, teach, get grants, and publish. This is especially the case for a potential "part-time" student, since research generally requires full-time focus to make good progress.

If you really wanted to pursue that path, I would recommend using your network. Do you have someone from your MS program who might be willing to write a rec letter to a colleague they know reasonably well? With a good introduction to pave the way, you are more likely to at least get a first conversation. Another avenue would be to get involved in your local chapter of your relevant academic society like IEEE. That might take some time, but would eventually be likely to lead somewhere in terms of opening doors to local researchers.

Finally, even if you don't get accepted on your first try, it's not a waste of time to at least try applying to your program of interest. Can't you consider some of your current work experience as "research experience"? Surely your work as a MS-grad isn't purely rote data entry or something. Maybe you have some patents or other quantifiable work products that could stand in for publications? If not, would you be willing to accept an MS position at the same institution that might get your foot in the door for a later transition to PhD?