The best syringe food (in my opinion/experience, usual disclaimers etc) is Burgess Guinea Pig Excel, the one in the orange bag. Most pet shops sell it.
It contains everything your pig needs, in the way of food, sounds like an advertisement!
The other advantage is that it is much cheaper than other syringe feeds. It does, however, come in pellet form and has to be prepared for syringe use, which takes very little effort. I once had a pig who was on it for nearly 18 months. Currently, I have a pig who has been syringe fed with Excel since last December.