I found the Devcon at a local Cal's farm and ranch store. All of the glue companies manufacture a similar product, for example, the description of Loctite 480 Black Max sounds similar to the Devcon Flex. Look for something that is advertised to stick to rubber, that is flexible and that has high peel resistance.
Bondit 45TH sounds ideal but I am in a budget crunch right now, trying to save money to buy a place where I can shoot in my back yard, so I have not been able to cough up the $60 price. However, after you have scrapped a few strain gages, you will have more appreciation for the worth of a reliable glue.
I've been getting gages from . The part number is SG-3/350-LY11, 10 gages for $55. These come with bare wire leads, so you'll have to solder on one of RSI's connectors and figure out your own mounting system. Be aware that there is a right side up that is marked in tiny letters that can only be read under magnification. I don't know the spec for the connector so I have been getting them from RSI.
Omega also sells some gages that have teflon insulated lead wires. That might be handy, but I haven't tried them.
In theory, any gage that is 350 Ohms and rated for 5V or higher should work. Be aware that not all gages are 350 Ohms and not all are rated for 5V.