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I noticed in looking at the origin of Oretgocactus macdougallii, that it "grows in an escarped limestone stone rock top, practically deprived of trees and scrubs, at a height of 2000 meters" according to cactus-art.biz

I can't do anything about the altitude I'm at, but at least the humidity here is very low and i've got a good bit of air movement going for me (due to the fact that either my electric heater or A/C is running). Now all I need to worry about is what it's growing in. Or what it SHOULD be growing in.

So, considering that it grows in limestone, WHERE do I get the best limestone for it to grow in? Any benefit to it being of a particular size? I wouldn't think powdered limestone would be best for seedlings.

I have several points here:
1. Some cacti do grow in limestone BUT it's not a matter of preference, It's a matter of tolerance. This topic is debatable.
2. I assume you can get limestome chippings from home improvement srores since it is an aggregate for cement mixes.
3. Powdered limestone...a recipe for disaster. It will form a cake easily and harden as soon as it dries.

Yes they don't really need limestone, they simply grow on it because they are slow growing and can stand prolonged dormant dry periods, therefore can grow where other more vigorous plants that would overtop and crowd them out cannot. You often see pictures of cacti in habitat growing on little rocky outcrops among areas covered with normal vigorously growing plants.

A Brazilian botanist Marlon Machado says that cacti that grow on limestone only grow when they receive acid rain and immediately stop growing when the rock turns the water alkaline. Some collectors add limestone chips as a growth retardant to keep the plants "natural looking" and slow growing, so not too fast growing and bloated, but obviously they grow much slower than in a more acid soil.

You can often get limestone chips from builders merchants in the UK, but not all stock the small size needed. Don't confuse cacti growing on limestone with those growing on gypsum. Limestone is alkaline, but gypsum has a neutral PH.

"Applying lime to a soil will raise the pH. The other product we think of with respect to calcium is gypsum, a hydrated form of calcium sulfate (CaSO4 ). Gypsum is neutral in pH, and since it has no carbonate ion as part of its makeup, it will not neutralize acidity."

See:-

http://ralph.cs.cf.ac.uk/Cacti/Cactus%2 ... linity.pdf

I don't have a definitive opinion about that. But I've been doing some experiments with dolomite, which is a calcareous rock.
It showed to be very bad to Pachycereus pringlei. On the other hand, for Astrophytum (which, as far as I know, is a limestone dweling genus), results are good.
In the top right you can see the seedlings growing in pure dolomite. They maybe look better than the seedling in the others mixes. I really need to see how they will grow, but dolomite seems to be a good component of the mix at least for Astrophytum.

also, see more discussion here:
http://www.cactiguide.com/forum/viewtop ... ne#p330185