I smashed a mirror while plowing one night--backed up too close to a tree in my yard and it smashed the glass...
I tried Pep-Boys,they had a large selection of mirror glass in the "HELP" line,but none that were right for my mirrors..
I had a few spare mirrors hanging around,but they were ones off my 81 G-10 van,and the arms are slightly different,so the mirror head is tilted instead of being level--and they were painted,the ones on my truck are stainless--so I opted to not use those..the glass on one of the van mirrors (passenger side with the "objects closer than they appear" message) was loose and ready to fall out,so I tried prying it out,and it came out intact...I decided to use it on my truck..(better than no mirror)..
To secure it into my stainless mirror,I did what the previous owner of my van did to the drivers side mirror glass--I put a blob of J-B Waterweld putty on the back of the glass where it was glued,then used clear silicone caulking all around the outer edge of the glass...I figured if it held on the van's mirror over the 24 years I had it,it would be OK...
So far so good,except I have backed into a few things because I keep forgetting they are "closer than they appear".

..been keeping my eyes open for more spare mirrors at junkyards and swap meets,flea markets,but havent found any yet that were worth buying..either too expensive or they were rusted,or painted crappy looking ones..