You can find all SAP t-codes (not only MM) here.
There are more than 100,000 t-codes there. It has the search function (based on t-code name, description, and its program) and shows the SAP menu path(s) for each t-code.
There are also additional information and links about the t-codes. You can also contribute by adding additional information and link of your favorite t-codes.
If you don’t have access to any of the SAP MM training materials, you can use transaction SE16 and search table TSTCT This is the Transaction Code Text table. You can search by language, Transaction Code (including Wildcards) and by text wild cards.
Remember that searching for *Material*Maintenance* will produce different results compared to *material*maintenance* (upper and lower case)