Any idea on where I can find a semi-mount that looks like this? I am open to ordering online or from ebay if it's from a reputable company/dealer/etc..
Those are fairly easy to make, but I would suggest one major improvement to that design. That would be to make it with double prongs at each corner, (or recut the garnet slightly to give it small flats on each corner). If you measure your stone you'll see that it is smaller across the sides than it is corner to corner. If you're grabbing it by it's longest dimension and you happen to bump it so that it twists a bit, then the setting is trying to hold the stone by a shorter width and it can become loose very easily. This is particularly important if the stone isn't perfectly square.
The cost of making a setting like this depends on the type of metal used as well as the number and size of small diamonds used. In 14 K gold and using 1/4 carat of small diamonds you'd probably be looking at $1K or so. Bigger or more side diamonds = more $. Post some pictures of that garnet if you get a chance.