Where is the best place to buy poppy seeds? I've heard that many sold in shops are not fresh, and consequently are bitter.

Spice House another family member to Penzeys and less pricey
Just type spice house.com

Thank you, everyone, for the helpful advice.

This is curious because I know for a fact (I'm a garden editor) that bread-seed poppies or the poppy seeds found in your spice cabinet are, in fact, Papaver somniferum, or opium poppy seeds that remain viable for years, perhaps even decades. Don't toss those old poppy seeds out in the garbage, scatter them. Since they need to be stratisfied (chilled to simulate winter conditions), toss them on top of a late season snowfall right where you want them to grow. I promise, you'll have your own home-grown poppies blooming next spring...and seeds to harvest and use in cooking once the seedheads have ripened.

They tend to go rancid quickly. If you buy them in a market, after you purchase open it and give a sniff outside the store. If it smells rancid you can return them right away. You will know it smells off. I also find Penzy's always fresh for anything.

For all herbs and spices, you want a place with frequent turnover.
Yes, seeds can go bitter or rancid, or loose some of their flavor if very old (years old).
Ask your nearby WF market about the turnover in their spice section, and act accordingly.
For other than Penzey's online orders, I know of nuts.com and kalustyan.com
Agree with Tiberia about cold storage of seeds (because of their high oil content). Fridge or freezer. When I have the space or a large volume of a seed and won't use it up quickly, I put them in the freezer.

When I get my poppy seeds (also from Penzey's) I keep them in the fridge.

I personally love the blue poppy seeds I got from Penzey's Spice.