where do i get an I.D. for public transportation if you have a disability?

um appologies if this does not make sense but im just wondering if there is some sort of Identification you can use on the trolley that states you paid for a month to use the trolley. im 17 and I usualy don’t travel on public transportation unless im on mobility lessons however I have noticed that some people just flash some sort of I.D. i could be wrong since im visual impaired and I cant really notice certain things from far away.
But would i be able to get some sort of identification that states i have a disability when I use the public transportation system in sandiego? if so is there some where to go to get it?