Assuming that you want brand new and not second hand this site is as good a place to start as any:
http://www.totaldigital.biz/default.php
However you do not say whether you want LCD/Plasma or CRT. If you want CRT(although I am not sure there is such a thing as40" TV in this) you will find them relatively cheap because it is becoming outdated technology. Similarly and to the best of my knowledge 40" in LCD/Plasma is not the most common size.
However where you are most likely to get a good deal is to buy/look for a non HD ready TV. This is not because they are not any good it is just that HD Ready is the latest craze.
I recently opted for a Panasonic 37" Plasma(non HD Ready) and not only does it look great on the wall but quite honestly with digital reception the picture is fantastic.
Hope this gives you a good start.