Ok. I've not seen any SIPs for sale directly. As an alternative you can
write one. It's not difficult.
Doug Boling's book has a sample and has a source download, so it seems to me
that that is not only a freeware SIP, but one with source (as some
developers like to only use 3rd party stuff if they have the source). This
is also beneficial because while it provides a fully working SIP, you are
also free to modify it if it doesn't meet your specific needs. Of course
that would require some effort.
I did a search on Google and came up with nothing, which often indicates a
commercial one probably doesn't exist, or if it does it's not advertised or
reviewed anywhere. This took 20 seconds of effort.
A search for "SIP" on Handango gave me 23 hits, several of which are
promising. This took about 60 seconds because their server is slow.
Sorry I didn't provide you with the exact answer you were after.