Once drains out you could were the cami with soft forms in. Prior that Erilan make a lovely soft clip front bra (which I still wear) and more comfortable than the berlei one. It has pockets for breast forms. I had one fitted post surgery by a specialist but you can order online. So go with current back size and pick a CD cup should be good to start. Lots of lovely colours to choose from too. I believe they are sold at Myers and David Jones if you have one locally.
http://www.erilan.com.au/collections/mastectomy-bras-underwear/products/copy-of-amoena-frances-mastectomy-bra-nudeYour breast care nurse can help with this too and you can get soft forms from Erilan too as well as cami tops. Have your ordered your berlei free bra yet through your Breast care nurse or this site?...I would go up a back size from your norm...I still find mine too tight.