With the exception of some large format cameras which require dedicated tripod mounts, most cameras use a universal tripod mount. This means that a standard tripod will fit just about any 35mm camera out there. You can purchase a tripod for your Nikon D3100 anywhere tripods are sold.Can you use any tripod for any camera?
However, camera mounts are standardized and you usually can use any tripod with any camera brand. Note that many tripods have exchangeable heads so you can swap them (if tripod mount on your binoculars differs from tripod mount on camera). You can even buy a tripod and a tripod head separately.