Post-doc independent money/grant (astrophysics/computer science) in the US (travel expense/workshop)?
Due to their (traditionally) short terms, postdocs are usually limited in what they can apply for. There are NSF Postdoctoral Fellowhips in your area, as well as almost every other area NSF funds. The other funding agencies in the US (DoE, NIH, etc.) also have fellowships available for postdocs. You should search the Internet more carefully for other opportunities.

That being said, whatever grant is funding your work almost certainly has travel funding included in it. Have you talked with your supervisor or PI about the kind of travel you'd like to do? You should.