It would be impossible to move around in the water where i normally fish on the tees without one, whereas on other rivers they would be spend most time getting in the way - so it depends on the circumstances - if it helps you negoriate the river better then get one. I dont think theres anywhere particular to buy one, you could knock up a pole with a weight in the end or pay 100quid depending on what you want, but they all do the same thing.
Ive got the old two part telescopic snowbee one which i think cost about 25 quid at the time. If your river is rocky with snags i'd totally avoid one of those collapsible ones because when i gets trapped and you lift on it cos you need it......it will....err....collapse! and your in the drink!