It's possible to do with a little piece of clay also (Like Play-Doh), it's just more difficult to measure because the clay can deform while removing or measuring.
Just about any normal solder will work, you can find it at almost any hardware store. As Burns posted, just make sure you find something thick enough to actually measure with, if your squish is at 0.050", and your solder is 0.030", you're not going to measure anything. But you also don't want TOO thick. It's possible to buy solder 0.100" and thicker, and that's going to be a bitch to squish down.
If you can't find something thick enough in the electrical section of the hardware store, try the plumbing section, they often have thicker solder there.