Speaking as someone who applied for numerical relativity, it's a fairly small field, but it's become quite a bit more popular over the last few years since the detection of gravitational waves. What's also become quite popular, though, is data analysis and instrumentation, and that has many, many more people working in it because of LIGO, and it's become quite intimately tied to numerical relativity.
Here's a list of a few schools off the top of my head with numerical relativity programs and some associated faculty who may be interesting. Some of these may not all be good choices. Saul Teukolsky, for example, is quite old and nearing retirement, but there may be other faculty at Caltech and Cornell who are doing numerical relativity.
Prestigious to Elite:
- Princeton (Pretorius)
- Caltech (Teukolsky, probably some postdocs)
- Cornell (also Teukolsky)
Good to Prestigious:
- Pennsylvania State University (Radice)
- University of Texas at Austin (Laguna, Shoemaker)
Lesser-known to Good:
- Rochester Institute of Technology (Campanelli, Zlochower)
- Brigham Young University (Neilsen, Hirschmann)
- University of New Hampshire (Foucart)
- Washington State University (Duez)
Here are some schools I know of with people who are actively involved with LIGO:
Prestigious to Elite:
Good to Prestigious:
- University of Maryland, College Park
- Pennsylvania State University
- University of Illinois at Urbana-Champaign
- Georgia Institute of Technology
It's worth mentioning that these are just a handful of the schools with active relativity programs. Nearly every school with a relativity group has at least one person doing LIGO stuff or numerical relativity; there are only a handful of purely theoretical groups left (UCSB comes to mind).