Tell the bait shop that you want to piggy back an order on their next delivery. Typically they are wholesaled by the pound. Rosy Reds are just red/pink colored FHM. They usually are easier for predators to see, so they are an easier target.
If you get them from a bait shop, be very careful when you put them in the pond. You'll need to sort thru them to ensure that you aren't introducing fish that you don't want - common carp, bullheads, etc.
It'd be easier to tell with Rosy Reds - any fish not "colored", toss out.
If you were to do a Google search using "How many Fathead Minnows in a gallon pond boss" you would find the first search result would be:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=210910