You can sometimes get it at livestock feed stores, or any veterinarian. it comes in several forms, pill, liguid, and paste. You must crunch the numbers very carefully by gram weight, as it is administered at 50mg. per kg(2.2 lbs.). of body weight. I had 250 mg. pills from the vet, and the stuff DOES NOT desolve well in water at all for some reason, but no big deal!. I take a small shot glass, crush the appropriate amount of pill to a fine powder, and add approx. 2 to 3/10ths of 1 cc.(only a few drops) of water too it, and mix as much as possible. It will only mix so much, but when it is drawn-up into a syringe immediately after mixing, it all gets in the syringe anyway, so it is all good nonetheless. Draw it up with a thin rubber tube, thin eye dropper, or thin ball-tip stainless steel tube at the end of the tiny syringe(make sure not to inject any air into the snake). Take the snake and gently hold it totally upright, so when you pry it's mouth open, and slowly administer the liquid, the gravity helps it go down into the stomach where it is needed. Keep the snake in this straight up and down position for about 15 or so seconds, and genly take the tube out, and you are done. Repeat this process in another week or so(WITHOUT FEEDING any meals) until a week after the last dose. Do not worry about the snake not eating during this time.
For a 300 gram snake(very large compared to your hatchling), the dose is approx. 1/16th of a 250 mg. pill. Just make sure you do the proper math here, and things should be fine. One good thing about Flagyl, is it is very "reptile-forgiving" if the dose is missed by a little, but DEFINITELY try to get it right on as much as humanly possible regardless.
If you are not comfortable doing this, I would strongly suggest you take it to a vet anyway, and tell him/her to do it anyway. Tell them YOU will take responsibility, even if they won't if they aren't good with reptile medicine. This is very straight-forward, and is very time-proven. Just tell the vet you spoke to someone with 42 years of snake experience, only he lives several states away, so he cannot do it personally for you.. 
good luck!, ~Doug
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"a snake in the grass is a GOOD thing" 
my website -serpentinespecialties.webs.com