Hi Annwyn,
I've always used a little extra support to fill out a bra, over here in the UK the girls referto them as "chicken fillets" , as thats what the silicone ones look like.
Before I go too far though, have you considered a gel filled bra. GG's use them as well as us, and they are a lot cheaper and easier to buy as you will already know your bra size. Over her I have used a brand called "Ultimo", but I know most manufacturers make at least one version. Prices should be from $30.
OK...back to "chicken fillets"...there are a number of different versions on the market, and they vary considerably. If your easily embarassed then these things can be a little tricky to buy, as you need to check the shape, size, and hardness.
The good news is that as there is a wider market with GG's, they are generally a lot cheaper than breast forms. A good set should not cost more than $80 and they may be as little as $30.
Shape ~ There are a number of different shapes:-
Filler ~ Some are more like a thin wedge of melon and go under the breast to push up.
Full ~ Others are like a small breast form with a hollowed-out back that go over the breast.
Push-up ~ My preffered type is one that looks half way between a melon wedge, and a banana. They have rounded front and curved back, and are best positioned down and to the side to give both volume and clevage
Hardness ~
Harder ones work better if you have got some good breast growth to work with, and they wil push up and inwards what you have got, to produce a good cleavage . The downside is that they are more visible under lightweight bras, and can look un-natural if not carefully positioned. The wedge "push-up" ones are often this type, which means they also feel less natural to the touch.
The softer ones tend to be the full breast, or the under breast "fillers". These will more easily take the shape of a bra, and will feel more natural to the touch. The main hazard with soft ones is something known as "pop-out" if you bend over with a low cut top (especially if you have been dancing)! These things love to escape to freedom given the chance!
And finally...All breast forms get a little uncomfortable on a hot day...

Hope that helps

Chrissty