How to find grant proposal samples?

Your best bet is probably to ask your colleagues if they would have you let a look at their (successful) grant proposals. That way you can also ask questions.

Many universities have a unit devoted to providing support with grant applications. Often they will have a database of previous grants (from members of the university), which you can examine.

If you are writing proposals for NSF consider volunteering as a reviewer in your discipline. This way you have both the opportunity to see a number of proposals and discuss them critically with others in your area. It is not a short term solution as it involves a significant committment of time and energy, but in addition to helping with your own proposals, you provide a valuable service and gain the opportunity to network with others in your area.

NSF also has a number of workshops on grant-writing that may help, but you likely will not see actual proposals during the workshop. In my area, NSF hosts a conference every 18 months with a number of grant-writing or career development break-out sessions which may help with the proposal writing process.