I currently get my Praxair 99.5% Instrument Grade n-Butane, or mixes, from Apis Labs in Eugene, OR. They also ship.
I tested it for purity relative to molecules longer than C-4, typically referred to as Mystery Oil, and it measured 99.9% purity.
After a single distillation and filtration through a VICI Metronics polishing filter, it tests out at 99.995% free of any molecules longer than C-4.
When I extract frozen material, using -30C butane/propane mix, I don't usually winterize with ethanol afterwards, because the wax pickup is so low. You still pick up the wax caps from the trichomes, but significantly less from the leaves, because they are two different waxes.
The molecule on the trichomes is polar on one side and non polar on the other, while the wax on the leaves is strictly non-polar.