I don't know about the best site but I trust two sites...

Spectrum Laboratories, at...
Don't Let Your Job Go Up in Smoke with Products by Spectrum Labs - Urine Luck
for $35.00.

and

Synthetic Urine, Pass Drug Test, Quick Fix
for about $28.00.

You only get one chance so I really trust Spectrum Laboratories for a few dollars more.

JMHO, Still Winston....
The downside to ordering the QF directly from Spectrum Labs is that Spectrum won't ship it to certain states. Persons residing in those prohibited states can go to detoxforless.com. They can ship the QF anywhere in the US.