If you're willing to come to Japan to buy your teapots, I can tell you where are my favourite shops/markets. What you ask would be easy if we were all in the same area. Maybe you can indicate in what city you live, so if someone lives in the same place, he/she can tell you where to go.
In the past, there have been a number of threads about different sorts of teapots on this NG. Good advice were given to evaluate yourself the quality of a given pot (as even in one store, 2 similarly looking pots can be inequal)and its specific usage. You should browse the archive before going to shop or ask specific questions.
Kuri