The recommended option is to install the latest LibGit2Sharp NuGet package , which will take care of installing everything in the correct location.
However, if you prefer to handle it manually, GitHub repository includes x86and amd64version git2.dll. Performing a branch check masteror vNextshould get them.
Remember that each version of LibGit2Sharp works against a specific version of the libgit2 binary. Each LibGit2Sharp branch contains a submodule libgit2, which indicates the completion of the libgit2 commit.

In order for it to work perfectly, the LibGit2Sharp assembly expects the binary to git2.dllbe placed in a specific tree structure.
LibGit2Sharp.dll
|__NativeBinaries
|__x86
| |_git2.dll
|
|__amd64
|_git2.dll