I know too well how u feel Kay! I also had a left breast mastectomy (with skin saving)..although recon is now off the priority list again for a while.
I found the silicone prosthesis heavy also and use my 'softie' that came in the "My Care Kit" for when i am exercising as it doesn't follow at 1/2 a second behind the rest of my body like the silicone does!
I too only have about 3 bras (mastectomy) but at least they are pretty and comfortable ( i got them from a store in toowoomba) ..so i guess my only suggestion would be to make some 'softies' urself and just have a silicone for the bathers/swimming...maybe u could arrange the BCNA to sell u some of the ones like in the "my care kit"...
good luck!!
Sam :)