I used red RTV silicone gasket material, but I'm not sure that's what would work best for your application. Are you trying to secure caps together or secure caps to a chassis? If it's the former, this should work but I'd first put o-rings around the caps to act as spacers and then fill the void with whatever adhesive you choose. That way you're not at all concerned with the compressive strength or elasticity of the adhesive. As for sources try automotive-type (Pepboys, etc) stores, it was in the automotive section of my local hardware store. If it's the latter I wouldn't use any kind of adhesive and would look for a mechanical connection like Pete, Kevin and Naaman suggested.