If it's dental try the UMKC School of Dentistry. They have a free or low cost clinic to train their students. Whether it's in session in the summer or not, or how often, requirements to participate, etc., I don't know, but perhaps they can reach a prescription and then give you samples. If you have a dentist they might do that, too, and there are free clinics but they are in high demand and you have to meet income guidelines.
There are other ways for antibiotics, but you're playing with fire and I wouldn't recommend it. For one thing even with antibiotics you have to know dosage, delivery, purity, side effects, and a host of other considerations. Some people can figure all of that out, especially with the internet today, but I would bet darn few could do it right. For example, you can probably find a farm and ranch stores selling livestock antibiotics such as penicillin, terramycin or who knows what else over the counter, but dosage, suitability, and delivery, etc. still play in as you would have to adjust it for human consumption.