Poppy seed might be in a health food shop, I don't know. If you eat too many poppy seeds and then have a drug test done, you will test positive for opium. They're not a drug, but they come from the same plant. I suppose that could be the reason you're having difficulty finding them. They also have a very distinctive flavor and I don't think another seed could be substituted without adversely affecting the flavor. I'm not sure what leaving out the butter flavor would do. It would probably change the taste slightly, but it shouldn't affect the consistency of the batter.