Nicoleandmaggie asked this question:
How do you deal with funding gaps (or potential funding gaps) if you have to support PhD students with grant money?
This is always unpleasant to think about, but very important. The short answer, in my experience, is one or more of the following: a) discretionary money (from an endowed professorship or some equivalent-ish internal award), b) small internal research grants (again, if this is available in your field and at your institution), or c) teaching assistantships. Teaching assistantships may or may not be abundant in a given discipline, but even if your department doesn’t have many (or any), there are related departments that might; for example, if you are in chemical engineering, your students might also be able to TA in chemistry, biochemistry, perhaps intro physics or biology; if you are in economics, your students are likely able to TA intro math or statistics, too. In my experience, anyone who’s needed a TA has ultimately been able to get one somewhere, but I don’t know that this is the norm, and it’s stressful for the students because it typically needs to be handled anew each semester.
The thing with funding lasting three years and the PhD lasting roughly five years is that there’s no guarantee a student will both be able to be funded the whole time and stay on the same project. Senior students typically prefer to teach and keep their ongoing project over getting moved to a new one three or four years in. Who can blame them?
As an aside, I think teaching undergrads is an important part of professional development for graduate students, and should be part of every PhD experience. Graduate student TAs also benefit undergrads, who like learning from instructors close in age.
Blogosphere, what do you say? How do you handle funding gaps if you have PhD students to support?
xykademiqz 
Math tends to be way less funded than econ, so they don’t tend to want econ TAs. In fact, it is usually the other way around– I have occasionally had Stats or Sociology TAs for my stats classes. But interdisciplinary departments (business school, policy school, school of X) are probably possibilities for getting TA work outside of economics.
n&m, over here the availability of TA is mostly about whether the department teaches large service courses to (mostly) freshmen or not. Departments that do that (generally in the L&S) have a ton of allocated TAs; those that don’t, don’t. Which is why, say, some engineering or econ or CS majors will go TA in math, but usually not the other way around, even though, as you point out, grant money is much more abundant in engineering or econ than in math.