It depends. Stock costumes may vary a lot in size and cut, you have to try it on. Depending on the cut of the bra (and the overall built of your body) the enhancer could either work or not. The end result should be still in proportion with the rest of your body.
For example, mine is small b cup (almost a), but with thick bones, wide shoulders and ribcage I can get away padding up to a large c. If the cups are bigger than that, it would look unnatural.
For padding I've used both foam and gel. The important thing is that the pad should be secured in the bra. (this is why I prefer foam, even shoulder pads, because you can contour it anyway you need and just stitch it in place; for the gel, you should make pockets within the bra lining).
Stock costumes almost always have to be fitted. It may be good idea to buy from a vendor that offers fitting as part of the service or for a small fee. (although come to think of it - each time I both from such a shop, after they fitted it, I still needed to do additional fitting myself; but it was still easier than doing everything myself).
Custom made can be a good idea if you know exactly what you want in terms of cut and style.
Hope this helps and good luck with the shopping
(hope i'm not reposting this, i had a mishap with loading the page while trying to post)
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