I most often use a mix of 3 parts white silica sand (about 1mm in diameter) and 1 part peat. The white sand makes it a bit easier to find small tubers once you need to do so.
For species with larger tubers I sometimes use a similar mix with larger sand grains (2-3 mm), but that is mostly for convenience and does not come with a particular advantage. The sand I use is available - at least in Germany - in shops offering supplies for aquaristic purposes and I use it without any pre-treatment (washing or anything like this).

I usually add a top layer of pure sand (again the same white sand I mentioned above). It certainly looks good but it has a very practical reason: I grow quite a few species which are self-pollinating and spend some time and effort on pollinating some flowers which are not self-pollinating. Although I try to harvest the seeds before they get scattered around, some seeds will always end up somewhere where they do not belong. To keep my plants from mixing up I replace the top sand layer each year and thereby remove all the seeds which may be waiting there for the next winter to come.

If you need the substrate with a certain moisture, I found it to be easier to mix moist or even wet peat (depending on the intended final moisture) with dry sand.