Highly recommend buying a quality brand. In testing most brands contain NO WHERE NEAR the dose they claim on the bottle or use low quality extracts or low bioavailability versions of the supplement.

The brands I recommend vary according to the specific supplement. And some supplements it just doesn't really matter what you buy too much, but that's the exception not the rule (ex. Vitamin D + Vitamin C are usually accurately dosed and of reasonable quality no matter the brand - though different formulations with some possibly limited advantages are available).

For quercitin I'd recommend

Nootropics Depot

Quercitin is substantially more expensive in high bio-availabilty forms - and is no less effective to just take a larger dose of the standard product. (That's not always true - it's just true for quercitin)

Nootropics Depot is the cheapest/mg brand I've found that has reasonable proof of quality and dose.

Life Extension is also good but more expensive. They also offer a high-bioavailibilty version - but as I noted at least with quercitin it's equally effective to take a higher dose of the plain ol' version.

Both brands I'd recommend in general, though there are other good brands as well.

Both fairly inexpensive on Amazon or their website directly.

I'd recommend avoiding the major brands you see at the grocery store for almost every product, for various reasons - mostly to do with dosage accuracy and product purity.

The supplement world is FILLED with bullshit. And it's also filled with bullshit by bullshitters claiming they're the one legit company you can trust.

Be careful who you follow.

3rd party testing is good. 3rd party testing with public testing methods is better.