You could buy Panacur from some farm supply stores, but because they sell it in different forms and I doubt you know the particular dosing for a leo of your leo's weight, so I strongly recommend going and seeing a reptile/exotic vet. They can tell you what the leo has and treat it according to that, because it could need another medicine altogether.
A couple of weeks ago I called a few vets for my sick leo and went with the one who seemed to know the most and treatment has been very successful. My leo had a pretty complicated problem though and wasn't eating... you could just as well call and find the cheapest vet. Buying Panacur from a place will also mean buying it in bulk, so there won't be a big difference in price since the vet will give you a small amount which shouldn't cost much at all.
If you pay $35 for an initial visit, plus $15 for a fecal, and $6 for medicine... it's very much worth it once you find your animal healthy. In my situation, I paid $40 for the visit but couldn't get and didn't need a fecal. Because of what my particular gecko had, it was put on three medications so that's $18 for meds... well worth it, though.
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Got hips like Cinderella / Must be having a good shame / Talking sweet about nothing / Cookie I think you're Tame 