Where is the best place to buy hardwood flooring?
Are the big box stores ok?
Flooring stores?
Do the warranties really matter?

A close out price on flooring will be your best. No warrantee on close out flooring. Following the directions that come with the flooring will make a warrantee unnecessary if it goes together easily and you understand how to lay the floor.
Retail wood flooring stores may be the best if you are only laying one floor. You may get the equipment, advice, and be able to return extra full boxes or bundles.
If you don't follow the instructions, the warrantee is worthless.

Depends upon what you want. Decide what species and style you want and then find a supplier. Not every seller will carry the same products. You'll get what you pay for. Do your homework and check reviews. Better flooring will have better milling. Buy the best stuff you can afford....hardwood flooring is kind of a one-and-done project.

You can search on online to find it.Take your time and be through in your search. Ask for recommendations from friends, neighbors, business colleagues, lenders, suppliers and subcontractors.

Expect to get the run around on any warranty with a box store. ugh

Depending on what you want, solid on site finished many contractors will supply or a local mill depending where you live.
I kinda like Uptown Floors myself unless your looking for a thinner 3/8 to 5/8 in prefinished.
I typically use Prosource for prefinished engineered floors. You can use my name and tellem your working with me as there are many Prosource stores across the country. Prosource is not generally open to the public and carry lots of name brand floors.