There are a few options for finding dissertations:
Rice dissertations are available online through the Rice Research Repository (R-3)
Rice dissertations in print, prior to 2012, when print ceased as a requirement for graduation, are located in the Library Service Center (LSC). To see a listing of all Rice dissertations, go to https://library.rice.edu and search with your terms. You will be able to limit to dissertations with the left-side option of "dissertations."
ProQuest Dissertations and Theses is a database of Ph.D. dissertations and Master's theses from the US and Canada. To access it, click on the "Databases" link on the library's home page and type "digital dissertations" in the search box.
OpenDissertations is an index of publishing information on doctoral dissertations accepted by American universities, with links to the full-text dissertation where available.
For dissertations where full-text access is unavailable, you may use Interlibrary Loan.