We were in a similar situation in the last YC funding cycle. 5 founders, 3 were not willing to quit their day jobs or move to Boston for the SFP. It absolutely is a problem - the first thing PG shot back was "Too many uncommitted founders!", and we didn't get accepted (or even interviewed). It'll likely be a problem without YC too: you end up spending more time ensuring that everyone's on the same page than actually building the project. We sped up noticeably after we dropped from 5 founders to 2.
I can't really help find a new cofounder (other than reiterate what others have said: ask around here, check cofoundr.com, etc.), but I can tell you what we did. We (my surviving cofounder and I) bought out the other 3 founders for $1000 apiece. In retrospect this was probably too high, given how quickly they accepted. But it was done mostly to preserve good relations and make sure nobody felt they were being cheated, and I think it succeeded in that. When our incorporation papers come through there will be a few agreements for them to sign to make sure we own the stuff they did before we bought them out.
I can't really help find a new cofounder (other than reiterate what others have said: ask around here, check cofoundr.com, etc.), but I can tell you what we did. We (my surviving cofounder and I) bought out the other 3 founders for $1000 apiece. In retrospect this was probably too high, given how quickly they accepted. But it was done mostly to preserve good relations and make sure nobody felt they were being cheated, and I think it succeeded in that. When our incorporation papers come through there will be a few agreements for them to sign to make sure we own the stuff they did before we bought them out.