If the offer has been accepted, then the best scenario is to attempt to negotiate with the sellers.
Other options include coming up with the money or backing out/defaulting on the contract. Check with a lawyer on the ramifications of doing so.
As for coming up with the difference, it partly depends on how much. In any case, though, consider friends and family. Or ask your Realtor to lend you his/her commission, secured by a second on your property.
Hope that helps.