A few possible sources:
- Ask friends and colleagues. Many are happy to share their proposals in confidence.
- Find out if your institution has a grant development office - and if they have a collection of old proposals to look at. Some of them maintain collections like this.
- Following up on #2, find out if your institution, department, etc. has a group engaged in "proposal red teaming", wherein you review in-development proposals from your peers as if you were an external reviewer.
- Try to become a proposal reviewer - for internal grant projects, or NIH/NSF study sections, etc.
There are also some groups that have been using FOIA requests to see grant proposals written at public institutions. Doing that is extremely controversial, and while legal, likely falls in the "bad academic citizenship" category.
Related question: maybe I am naive, why don't NSF and other funding
agencies publish the full proposal of the ones that were accepted, say
5, 10 years after the funding ends? It seems to me that these older
projects should have wrapped up already and there are probably minimal
risk of being scooped.
These proposals often contain long-term or overarching research plans that may not be complete in five years, unpublished data, etc. My guess is its easier to simply not than to try to parse that out and/or redact that information.