There are a few places.Eser is good I got a punching man from them(They are a bit expensive)-
http://www.eserlanka.com/
Also u should try lal Chand(They are oppersite abans in kolpity)
There was also a place in kalaniya cant remember the name right now.They make benches and stuff for order.Also there a whole lot of small sporting good shops all over S.L
One place I can remember is a shop in Maharagama in a shopping complex called amity.I got my weights from them.