It doesn't need to be a really bright star, you can use any star as long as your guidecamera can see it clearly. Avoid optical doubles if you can.
Losmandy rings come in the range of 90mm, 108mm, 125mm and 160mm so I imagine other brands would be similar. If you are handy with a welder and a drill you could make your own. This
page will give you an idea of what your after. Or have a gander at
Andrew the Astroman's homemade set