three ways come to mind here...
If you have a source for what you need but in gold, simply cast
duplicates in sterling.
Instead of rounding out a triangle, try making a curve with your
flat wire that matches the stone, then mark off and bend the corners.
If you use these a lot maybe think about making your own bezel
mandrel. It would indeed be a lot of work but if it gets the job
done, might be worth it. If you started with a commercial triangular
mandrel you'd need a good grinder and a steady, consistent hand. You
may not need to do the entire length, just the size range you use
If you opt for using the available bezels in gold, for a two tone
look, 18K will make a richer statement.