Any platform for building research collaboration? My research is theoretical and have published some papers in high impact journals. However, I need further expertise to advance my works (e.g., advanced mathematics). Is there any online platform to post my research idea on which like minded researchers express their interest? 

I look for a platform in which people are frequently doing this. I know it's somehow possible in websites like researchgate, but it shouldn't be very likely to happen. 

No funding is involved. I do most of the research and write the paper, I just need a mathematician to do the math formulation or even check my math works.
My advice is to use personal, in person, (sorry that this had to be repeated but it did) outreach.  Network and find someone at your institution and talk to them.  If you are not at a strong enough institution to have anyone, then network outside the institution.  Forming collaborations is an important and valuable thing.  But doing it on a board is a bad, bad idea.