Local winemaking suppliers carry carboys in these sizes (U.S. gallons):
2.8, 3, 5, 6, and 6.5 (roughly 10.5, 11, 19, 23, and 25 liters). I can't seem
to find any different sizes from online retailers, either.

I've started a couple batches of fruit wine (one pear, one apricot) that
aren't going to fill a 5-gallon carboy. The pear is still in primary, and it
looks like I'll have about 4 gallons after straining out and pressing the
pulp. The apricot is in secondary in a 6-gallon carboy, but there's at least 4
inches of sediment at the bottom, maybe 5 -- that isn't going to fill a
5-gallon carboy either.

Does anyone know of a source for 2- and 4-gallon (7.5 and 15 liter) carboys?

TIA...