I don't know of any source to buy less than 1000 doses of the Mareks vaccine. Jefferspet.com, stromsbergschickens.com, and Valleyvetsupply. com are some of the online stores that sell vaccines. The vaccine itself is not that expensive, but it must be used all at the same time. You will have to buy ice packs and a cooler for shipment, and that with shipping can boost the price a lot. Some have said they have seen it for sale in feed stores, but I have never seen it available in my local ones--TSC, Rural King, Southern States, or locally owned ones. Just Google Mareks vaccine for sale to get more online store info. I usually have chicks vaccinated at hatcheries, but the ones that I have hatched at home have never been vaccinated. We have never had a case of Mareks thank goodness.
I'm hatching chicks next month and have healthy hens, but had a 15 week old chick die of what acted like Marek's last year. She was ill for a couple of months before I had to cull her and I've read the virus stays in the soil for a long time. I'd hate to hatch healthy chicks and have them contract the virus, but I might have to take my chances if I cannot get ahold of the vaccine.