I think it might be the ones at the back, those these might be the hobbycraft ones...
http://3.bp.blogspot.com/-loWW9R0YD9k/T ... e%2521.jpgI couldnt find the exact blog post that says exactly how he made the terrain, which I know he said were the cactus was from.
Sadly irregular still live in the 90's, the good old days of black and white catalog miniature ordering! However I have taken many chances with their stuff, and though not as good quality of CinC, GW, onslaught etc... they do have a very large 6mm range, and almost ever thing I have ordered from them has been just what I expected or better, and even some that were beyond excellent! (their ratman range fro instance)
If no luck from irregular or they are too large (depending on what you will be using them for) Hobby craft use to carry a range of very small cacti for doll houses. There were pricey when brand new, but I did see them on sale for dirt cheap a year ago. Maybe even try an actual doll house store.