Some useful info and suggestions
here.
The common sizes are 25.4mm (SR, Nitto & ITM), 26.0mm (3TTT, Cinelli +'95) and 26.4mm (Cinelli -'95) so you might struggle to find 25.8mm. Why only 25.4mm or 25.8mm clamp sizes? Do you already have stems that size?
Provided the clamp area of the bars matches the clamp area of the stem, it makes no difference whether the stem is quill or Ahead, the bars are the same.
If you are considering an Ahead stem, presumably you are thinking of using a quill-to-Ahead converter? Usually this is done to allow the use of oversize bars as this is the standard for Ahead stems; if you use a converter but stick to "old" sizes, you are limiting yourself somewhat.