For the creme fraiche I just put some in UHT milk and let ripen for 24 hours, similar to making mother culture with DVA starter. However, I didn't get "cloggy" milk as a result, so I'm not sure how active it was. Also, it appears creme fraiche uses the same basic cultures as buttermilk anyway (just put them in cream), so just going with buttermilk should be fine. If you want to use it, rather than make cubes, just put a table spoon of it in some warm milk, say a cup or so, stirr to break it up, and add it in when you add the buttermilk cube.
B.Linens can be purchased at any cheese making supply store that has more than just kits. It's a pretty basic rind development ingredient. Or, you can just wash the cheese with a cloth and 3% brine by weight (i.e. 3 g salt, 97g water: total 100g solution, 3 g of which is salt). Wash the cheese with that, and keep the cloth in the cave. Don't rinse the cloth, and keep tipping the brine onto the same spot. B.Linens are naturally on your skin, so they will transfer. You can wash the cheese with your bare hands too (wash with regular soap, but not anti-bacterial) to help transfer. Wild b.linens will grow if you keep the cheese surface friendly to them, which the salt wash should do. You could add a splash of white wine to the brine as well, though I've never done that myself.