According to Mammoth's city council, 54% of the housing pool is vacant 85% of the time. People don't rent out their vacation homes and when they do it's AirBnB not long term to 7 young adults. So, I'll preface this by saying that there's a very high possibility that you will not be able to find a place in Mammoth.
https://www.blizzardpm.com/vacancies/ is the biggest long-term property renter in town but they don't allow cosigners and are very strict regarding credit checks, employment, and rental history for all tenants. If you all aren't squeaky clean, don't waste money on an application fee.
http://classifieds.mammothtimes.com/ca/rentals/search local classifieds
https://www.facebook.com/groups/1665187730466716/ facebook housing group
Problems you'll have:
1. There's not much long term housing
2. The long term housing available is very difficult to get and usually expensive
3. Housing requires a minimum of a 6 month lease most want 12
4. You're very young
Good luck, don't get your hopes up.