I think you can find a range of fluorescent probes with different prices to order online if you just search keywords such as "mitochondrial dyes"; however if you are looking for a very specific one to synthesise yourself, you need a strong background in chemistry and focus on organic luminophores or nanoparticles (mostly quantum dots and rode-like nanomorphologies) that can actually penetrate the cell membrane and react selectively with your analyte (mitochondria).
According to the very good Ramin ´s answer, please tell us (the answerers) what a kind of cells are you using (or will be used) and the conditions of staining on your plates (e.g. growth medium a.s.o....).
According to the fluorochrome and its structure and excitation (please choose the correct filter), please note that fluorochromes are toxic for cells after a lot of time.
Well, this is a tricky problem to solve! As Ramin already mentioned, there are several mitochondrial dyes on the market - the "Mitotracker" series is likely the most popular. However, all mitochondrial dyes that I am aware of stain mitochondria due to a membrane potential across the inner mitochondrial membrane. You have to consider that in case of "damaged" mitochondria, which may exhibit a reduced membrane potential, the amount of dye bound may be reduced and give a wrong information about mitochondrial "abundance".