Marimo are super easy to take care of! They prefer to live somewhere relatively cool and dark, so after taking their photos in the good light by the window, I have found them a nice little spot in one of my expedit cubby holes.
They are quite happy in tap water (most tap water shouldn't have so much chlorine in it that they will mind) and need it changed once a week or so. Every so often, it's good to give them a clean and reshape by gently squeezing and rolling them. (Normally, they'd keep their shape rolling about on the bottom of lakes.) I did this when I brought them home, and wow they are so squishy and soft! I think I have a problem with anything that vaguely resembles a tribble, and these little fluffballs are baby water tribbles for sure!
The glass bowl was from Asda for £4, the glass pebbles were also from Asda at £1 a bag (I used 3), the fake purple plant was £2.50 from Pets at Home, and the marimo (also Pets at Home) were £3.99 each. So, the whole thing was under £20, which I think is pretty great, and it looks super cute!