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. 

 I might have to give this a go. It sounds like fun to learn and experiment with and should work out cheaper than the ready to grow bags. 

 It's definitely fun & interesting.

You can keep it pretty cheap but you'd want to be growing a few bags to make it worth it.

A huge shortcut would be buying colonised grain spawn bags ready to inoculate your growing medium. At this final stage, contamination is less of a risk because you're dealing with less nutrient rich medium.

Cooked grain & sugar water is so easily contaminated.