Any Auto Parts store in America.
Go to the Help aisle. The most commonly stocked T fittings are sold under the HELP name (Motormite), or actually under the Vacu-tite name. They make assortments of any size you could ever need, and they're only a couple bucks. Once you determine the size you need, you can apply a little bit of silicone around each end, to ensure it seals properly. They also make some rubber versions, that seal even better w/o having to use the silicone.
It's easy stuff.