The easiest way to to keep an extra filter on another of your tanks running. If that is not an option, keeping an extra filter pad in one of your HOB filters will give you the ability to pull one out and start it on the QT tank. The only bad thing about this route is that if you have a disease outbreak in your main tank, you can't use that filter media on the QT tank without risk of spreading disease to it.
A few friends of mine that keep multiple tanks keep QT tanks going by keeping a few large apple snails in them when they are not needed. They say its just easier to grab the snails out and put them in one of their other tanks if necessary to make them empty for the fish needing quarantined.