Edit: as ATpoint mentions, you will require either a reference genome FASTA or reference cDNA FASTA depending on what you are planning to do.
You can obtain this from GENCODE: https://www.gencodegenes.org/mouse_releases/reference_releases.html
The latest releases for each build are shown on that page.
- GRCm37 = mm9
- GRCm38 = mm10
Other releases can be accessed via the drop-downs / tabs. You can download both GTF and corresponding FASTA files,