Most mpreg fics tend to be slash. So when you find one, it will most likely be Spike/Xander or Spike/Angel. That's not quite what we do here and I couldn't point you in any direction anyway because I don't particularly care for Spike being in love with Angel or Xander (especially Xander), so I don't go near it.
However, there are a few fics that are Spuffy or a non-ship that I have stumbled across throughout the years:
Check Mate by Xela is located here on the Spuffy Realm.
Seraph by Snickfic is also good. There's no ship, per se, but there are a few tender Spuffy moments and I could have easily pictured it going in that direction.
Snick also has the To Come Home To 'verse, which is Spuffy.
I can't post links because I can't remember my username/password, but you can located Snick's fiction on her livejournal.
Type: snickfic [dot] livejournal [dot] com.
There should be a masterlist there at the top of the page. I know she also recently did a het mpreg ficathon that might have some Spuffy. I don't quite know what's on it though cause I don't read a lot of mpreg. | 
