I’ll agree this can be a problem. When shopping for breast forms (particularly online as I do) it’s important to take your time and do your research. Stay away from dome-looking forms as they will probably not look natural. Sizing as you’ve said is a concern and lately I’ve taken to looking at the actual measurements, holding a cloth tape measure to my chest and mapping it out. Color matching is also a big concern for low-cut tops.
In general for natural-looking breasts you’re looking for a form with a low profile. You want a big footprint and a proportionally low projection, particularly if like myself you have some natural breast tissue. Ideally you want the forms to meet in the middle and still have the width to partially wrap around the side.
Finally, in general you get what you pay for. I recently splurged on a pair of Aphrodite forms and other than a slight color mismatch I’m well pleased. The only thing I might do differently is go a size smaller.