There are many inexpensive T-mount adapters on the marketplace, but if you want one that works really well you should consider one from Vixen.
Vixen's T-Mount SLR Camera Adapters are meant for use with their various methods of mounting SLR cameras to the rear of their telescopes, but obviously they can be used for anyone who has a T-mount system lens and wants to mate it to their SLR camera without any real trouble. Many, if not most, of the inexpensive T-mounts will get stuck to the camera or lose adhesion to the lens; with the Vixen models, the superior workmanship really limits the incidence of these kind of annoyances. Depending on how much you use it or how expensive your camera is, it may be worth springing for the extra quality that Japanese manufacturing delivers.
Please note that on many modern DSLR cameras, the T-mount lens and T-mount combination will only work on the camera with the camera in the "M" manual exposure mode. Additionally, the most advanced metering modes usually don't work with the T-mount.
In the Box
Vixen Optics T-Mount SLR Camera Adapter for Olympus OM System Cameras