Where can I post questions I want to ask my fly research colleagues (such as to get help finding a fly stock or protocol)?

There are severals ways to ask questions of fly colleagues.

Bionet.drosophila provides a discussion forum and posting site for Drosophila-related questions, events, and job postings.
FlyBase General Forum lets you ask questions about FlyBase tools and data.
FlySlack connects fly researchers to one another on the Slack platform.
Other ways to reach 'fly people' with research-related questions include posting questions at Twitter (#Drosophila) or asking questions at ResearchGate.