Use any clean bucket. I find the semi-transparent Rubbermaid tubs works esp. well. Throw in a heater & a filter & you are good to go. <And if you keep the filter pad in your main, then you can use that to get an instantly cycled setup at a moment's notice.>
And when not using as a QT, my Rubbermaid tub (30 gal) holds all my spare fish stuff.