In the US, for good rubber and similar cords, look in the yellow pages
under gaskets and ‘O’ rings. It is very inexpensive compared to buying
ready made cords, although one usually has to buy a minium. There is
American Gasket in Houston, for example. I was told that buna is
actually better than rubber, because rubber can harden or stiffen
faster. You can get just about any mm. size you need. For fasteners,
make them out of wrapped wire with loops on the end or buy the crimp
clasps found in jewelry supply places, such as Rio Grande.