Yeah, I know. But it doesn’t seem worth it to buy a kg of flour just to make yeast for a single recipe ;)
You can always ask around in your friendly neighborhood forum for a starter culture, there's a lot of people that are willing to share.
If you have bakers yeast, you can just feed it and multiply the amount trivially: Half a cube (20g) or 7g dry yeast, mix with 15g sugar, 100g flour, 100g water, let rise for an hour or two in a warm place. Fill in icecube form, freeze. Each cube is about half as strong as a "regular" cube, so you may need to experiment a little with rising time for your dough.
I did this last weekend and made a pretty good loaf (my first time).