First, make sure they are large enough for your torso, without effecting the Dolly Parton look. They should be the right shape, meaning that several pairs of socks will probably be too lumpy. They must also have the proper weight to look natural. The weight gives them the proper movement and helps them attain a realistic contour. Foam rubber will not do unless you only need a little padding to add one cup size to your natural assets.
Silicone is expensive, but probably the best.
You can make do with well-sealed plastic bags filled with water. Whatever your choice, be sure not to cinch up your bra straps too far. Breasts should go slightly below your armpits, not your collar bone. Your complete image requires that you learn move your body like a woman. The best way is to be coached by an expert.