Transmission oils (manual)

Where would I find this? Looking to change tranny oil this weekend, so it’s all prep’d up for track time. Also, since its synthetic, and I don’t know when the last time the previous owner changed his tranny oil, wouldn’t it cause harm to the internal parts?

Someone let me know, before I make a mistake.

Can I find these fluids at PartSource, CT? Or only performance shops carry them?