There's only one manufacturer at this size and that's Panasonic. And very good they are too. Apparently it's not economic to manufacture small plasmas. So if you want a tv smaller than 37" you have to buy lcd. If you want a choice of plasmas then you have to buy 42" or larger.
End of 2007, LG announced a 32" plasma: 32PC5RV. I havent heard anything after that. There definitely are no 21" plasmas available and will not be for sometime due to technological limitations and manufacturing costs are high.
There is a certain minimum size to economically make a plasma cell/pixel. It's only been in the last year or two that they've managed to squeeze 1080 lines into a plasma at 42". In theory that means they could just about make an SD (525 line) 21" plasma, for probably £6-700, but who would buy that?