Up here many farm supply stores carry them. If you just need a couple dozen, thats probably the easiest. If you want a larger amount or to do a tubing system, google maple equipment suppliers. for a few metal ones, often you can buy them from a sugar bush that has converted from buckets to tubing.
I don't do all that much. I use short sections of 1/4 inch clear tubing from the home store, about 8 inches long. I drill a 1/4 inch hole in the cap of a 2 liter soda bottle and run the tube through the cap and screw the cap onto the bottle. Then I use a short piece of scrap electric fence wire to make a hook around the bottle neck. I drill a 1/4 inch holee and inch into the tree and insert the tube approximately 1/4 inch. I screw a stainless steel deck screw into the tree so the bottle hangs just below the hole. I keep a supply of extra bottles and caps. When I collect the sap, I just unscrew the tube/cap and put a solid cap on the bottle of sap and then i place an empty bottle in it's place. Maybe a primitive way to do it, but it beats buckets and spiles Keeps dirt and insects out of the sap too. I purchased two inexpensive stainless steel 5 gallon kettles at a discount store and put them on an old kerosene stove one an enclosed porch. I cook down about 4 gallons of sap and as it cooks down, I add additional gallons of sap until I have evaporated ten gallons of sap down to a quart of syrup. When it gets down to only an inch or two in the kettle, it must be watched carefully or it will scorch.