Find articles about the language you're hoping to study and watch the citations in those articles. Often you'll find citations to a relevant corpus or dataset.

See the Corpora page.

1) Start with Glottolog and Ethnologue

2) Search Catalyst

In library searches, the word "Grammar" applies to two things: a core linguistics work that describes and provides examples of a particular language, and a school child's guide to learning a native language. There's no perfect way to distinguish between those two in your searches, but here's how to find what you really need:

1. Do an Advanced Search and put your language's name in one line as a SUBJECT.
2. Then put the word "Grammar" into the next line as a SUBJECT.
   
3. Then scan the results looking for works that were ideally published by a major scholarly publisher, such as a University Press.

This technique also works in WorldCat.

Limiting to scholarly publishers

If you're finding lots of grammars that aren't scholarly (maybe aimed at second language learners or grade school students), try limiting your searches to just the core, known scholarly publishers in linguistics. Here's an example of that kind of search for you to use:

3) On the open web

Many linguists publish their findings on the web, either in collections like OLAC, through organizations like SIL, or on their own researcher websites. Here are some of the best ways to find that scholarship: