My other idea is that you can buy implant sizers. This is what you use to decide the size of implants you want. Basically they are implant immitations. I tried them in many sizes to pick what I wanted but many women wear them for months to decide. It think this would be cheaper than prosethics if they were too costly.

Depending on the amount of difference in each breast this might work well. I have silicone forms ( like Big Mama mentioned) and they work well because you can also use them in a swimsuit without soaking up extra water. Swim suits usually have a liner..so I cut a small slit in the liner and insert the silicone form. It works well and no worry of it falling out or moving around. You can get silicone forms even at walmart for around ten bucks. Only thing is not much selection of sizes that I saw here.