How do I keep a jar of water with algae warm for a whole week?

Keep it on top of your fridge to keep it a fairly constant warmth. It will stay about 35 degrees if you keep it in the fridge. Sorry, don't know where you can buy it, but you can culture some by leaving a container of water in the sun for several days. Measure it by taking photos in the beginning and later. Hope this helps...

Don't bother buying algae -- you can get it for free. A glassful of pond water, kept at room temperature or a little warmer for a week or two, will grow large amounts of algae; lake, river, creek, or seawater will work just as well, and even puddle water or even gutter water *might* do the trick. If one of your friends has a well-established aquarium, ask him or her for a sample of the water or an algae scraping from the tank wall -- chances are that they've got a nice colony of the stuff going. (It grew all over every surface in my tank til I got an Otocinclus...)

As for warmth, you may not need to do anything you live in a reasonably moist climate (not a desert or semi-arid) and it's summer where you are: the air outdoors will be plenty warm. Use a thermometer to record your algae water temperature two or three times a day, and you're golden.

Failing that, there are several places you could put the jar. On top of your refrigerator, or wherever else it releases heat, might work. A sunny window works, but only during the daytime, and be very careful that it doesn't get so hot that it actually kills the algae. If there's a space heater in the house, ask if you can borrow it for a week: it might take some experimentation to find out exactly how far away you need to keep the jar, but it'll do.

If the algae grew, you'll know: it'll turn the water green or brown, and form a film on the container walls or filaments of algal material.

Happy growing!