I felt that a larger mirror would be a better choice and was specifically seeking the 3x5 mirrors.
After checking a few sources, (sporty's pilot shop was suggested) and simular, the only signal mirrors I found were the 2x3 mirrors.
Lest we lose sight of your original goal ...If you search for MIL-M-18371E, you should find sources that will sell you a good S.I. Howard 3"x5" MIL-M-18371E mirror (quantity 1).
I see three shops currently offering them:
Survival Resources - $17 + ? shipping
(John McCann sells lots of mirrors and seems to know his stuff,
so I'd buy from him, though I have not before)
http://www.survivalresources.com/Products/Signaling.htmlSupply Cache - $19.25 + ? shipping
(I have bought mirrors from them, long ago)
http://www.supplycache.com/Signal-Mirror-Glass-Howard-Glass/productinfo/900-30701/Best Glide (you need to use the dropdown for the 3"x5") - $23.95
http://www.bestglide.com/cgi-bin/cart.pl...p;search=SN1338The other option is Ebay and patience. I prefer the finer mesh aimers used in US military models from 1949-1984 myself, but I don't see any reasonably priced auctions on Ebay at the moment, though I've bought many MIL-M-18371A,B,C,D & E signal mirrors on Ebay.