Seems like it would be a simple thing to order a tie tack back, but there are points to consider.
The Tack - It has a head and a nail.
The head of the tack is the round disk. It is available in several diameters. 10mm is the most common but you can also get them in 4mm. If your gluing them, a 4mm head is almost always too small to adhere well. The 4mm size is often soldered on or maybe just used with nothing on it at all, like through a folded ribbon. There are also tacks with a small prong on the head, this prong is used to keep the end product from spinning around on the cloth.
Next we have the nail or shaft. If the application is going through a single layer of fine fabric you don't want a long nail as it will flop a bit. The nail length is also contingent on what type of clutch you are using.
There are several types of clutches. The ones most people know are the butterfly clutch or pinch clutch. This is the typical one you see on military metals and flag pins. The other type is called a spring clutch which you might see on fancier tie pins. The spring clutch requires a longer nail to work, make sure you are ordering the right tack length for the spring clutch. A butterfly clutch will work on any nail length but I recommend going with a shorter nail for most applications. For example if you think people will put the tack through a hat or on a lapel.