FYI
A freind and I have also been on the search for the elusive Southdown. We came into contact with this rare breed a number of years ago, and when it was time for some reworking of the tanks we started searching for it a couple of weeks ago.
We have come close twice in the last few days.
1st we found Old Castle, allegedly Southdown renamed and repackeged. This was found in a Home Depot in Westchester. Package says that there may be some silca, and has the "not for aquarium use" warning (As it is my professional opinion that no one should do anything for fear of liability, I take such warning with a grain of silca-I mean a grain of salt) Now the test - we have bubbles. But the inquisition does not end there... The grain size is much larger than SOuthdown and it feels a lot rougher when you run your hand through it.
Next we found Yardright, wich I had been told long ago was just as good as the Southdown. This was found in an Agway in the vicinity of Garden City. A bag is purchased, some vinegar is found and we have......... bubbles. But once again the grain size is larger than the Southdown and feels very rough.
As indicated each of these was compared to southdown. We had a few pounds left over from the last time we went looking for it (Before anybody asks me for the sand, when I say a few pounds I mean two or three pounds). When you compare them the grain size is much larger, it is still in the vicinity of being "sugar" size but it definately on the large end. Furthermore, the "feel" of the sand was off. Instead of being smooth and silky, it was rough and harsh.
After all was said and done, we deceided that we would just have to cough up the money and purchase aquarium specific (i.e. much more expensive) sand.