As others have said, conductive epoxies are the way to go. You can get one part (heat cure) or 2-part (resin + hardener, mix to cure). Just keep in mind they tend not to be very flexible, so if it's something that will undergo bending stress, look into a silicone base rather than an epoxy base. These epoxies work by being comprised of 99.9% pure silver balls suspended in an epoxy binder so that many of the balls are touching each other, giving conductivity. That's why it's so expensive, it's almost all pure silver.