Before I decided to vinyl-side our home, I too had to replace a few shingles that managed to get blown off in a wind storm. I bought the shingles at Home Depot but they didn't have the tack nails. I went to a Roofing & Siding place called Bass Supply and they didn't have them either. My shingles were nailed directly to the gypsum board on 2 out of 4 sides of the house. The front and back of the house had furring strips to nail to.
I don't see the shingles being used these days in new construction, thus, the tacks are hard to find. If your shingles are secured to furring strips, there are 1" 'cement nails' available. Just go to the Hardware Aisle in the Home Centers and look/ask around. From what your describing, the 'holders' you describe sound like what I have. Actually, they're not tabs, per se. My tacks worked like this? When you hammered them, the tack-points (yes the tack was more like a cotter pin) V-ed out to hold the shingle to the gypsum, and the head kinda flattened-out like a tab. I could be wrong in thinking this is the same type of tack that you have but good luck in finding them. If you find a source, plez e-mail me direclty so I can tell 15 of my neighbors who still have the shingles where to get them. If you want to try Bass Supply again, any WEB-based Yellow Pages can help you, or check your local White Pages. (If you use the WEB, don't enter in a city; just enter in the Business and the State.) If you don't find Bass in your local phone book, try the ones from your surrounding communities.