Ditto: Also these adapters will be found in irrigation (used for drip irrigation) and plumbing stores. Take the tubing with you to be sure it fits both ways. Anywhere that carries tubing ought to have a selection of barbed fittings, including adapters to connect tubing of different sizes. You may have to go to PVC and back again.
I use marine grade hose clamps or the plastic type that are sold with the Rena Filstar canisters. (Rena is not the only supplier, this style is also available separately)
Hint: to make the barbed fittings and tubing slide together easier add a few drops of vegetable oil to the barbed fitting.