I bought my Amprolium from First State Vet Supply online, and there was no minimum order that I was aware of.
Treating coccidiosis is a very personal choice, and each option has its risks, but you could try some of the old-timey remedies that seek to boost the chicken's helpful gut bacteria ~ with things like buttermilk, for example, or other probiotics like yogurt. While no cure is a sure thing, Amprol is sure dependable . . . but I have also had good luck with probiotics whenever I have tried them.
Good luck. Please keep us posted on your progress.