I did take a tissue sample from some white oysters I bought. I put that sample on agar, let it grow, transferred the inoculated agar to fresh agar (twice to remove some contamination).

I then used the clean agar sample to inoculate some sterilised sugar-water to grow liquid culture.

It can be pretty cheap to do your own bags, but you need a pressure cooker & a lot of patience. There's a learning curve to it all.