Wellcome to the forum!
We have a poppy seed grinder which looks pretty much like this one (first one I found on google):
It has a conical drums with grooves inside which presses agains the inner walls as you turn it. It works perfectly. You use the screw to adjust the pressure. Very easy.
And yes - most of the time one needs to grind poppy seed - they are like tiny balls and have much stronger taste and very different consistency once ground.
As to where to buy one - I had one problem to find plenty on ebay and surely plenty shops carry them too (may depend on your location which is unknown to us)