Either mini bikini cookie cutters or mini bikini molds?? To fit on top of a cupcake. So far the smallest cutter I can find is over 3". Not going to work..
If you need to make a lot of them, then the price isn't too bad. If it's just for a dozen, or two, it's a bit pricey:
I actually found some cutters by the bakery section of my grocery store a couples months ago. They are actually sunglasses, but they also work as a bikini because they don't have arms. They are about 1.5-2inches. They were made by Wilton too, not sure where else you could find them though I've never seen them at Michaels.
The second one is a set, in which the sunglasses could be used for the top, and the set is cheaper than the cutter in the first link.
For the top, you could find a strapless [wedding] dress cutter and just use the bust part, like this:
Or this one and cut off the middle part: