George

Sorry I'm late to this party.

Indium corporation makes and sells a bunch of low temperature solders.
You can search for them here:

http://www.indium.com/solder-alloy-guide/

Wikipedia also has a good list:

https://en.wikipedia.org/wiki/Solder

There is a trade off between temperature, the ability to wet a
surface, mechanical properties, and how hazardous a material is (i.e.
if it contains lead). In general, the low temperature solders aren't
anybody's first choice. They don't wet very well, and don't have very
good strength.

Thanks,
Chris