I've ordered from both of the following, and have been satisfied with the service. I haven't grown out the seeds yet.
howarddill.com
pandpseed.com

If ordering from howarddill.com, I'd recommend ordering sooner, than later, as it took a little while for the order to arrive here in MN. I think the seeds go through an inspection, before crossing the Canadian border.
If you want "premium pumpkin" seeds from Howard Dill, order soon, as a few are already sold out.

Douglas

Velvet queen is not a giant sunflowers, it's an ornamental red sunflower that is usually about 6 feet with multiple blooms. You can buy velvet queen at most garden centers that have a descent seed selection. Most commerical online seed catalogs carry velvet queen as well.

The common russian mammoth is sold by almost every seed company and can be found everywhere; from walmart, k-mart, home depot, etc. Russian Mammoths grow anywhere between 7 to 12 feet tall or more, depending on the brand.

Skyscraper is a commercial variety sold by Ferry Morse. It's a pretty good variety if you are looking to get a big sunflower. They usually average close to 12 feet. They aren't the biggest, but they are pretty consistent.