I'm +1 with the in tank thermometer, I have a sticky one right now, because they're what I use for my snake, but I don't think it's very accurate and am getting an in tank one soon.
If you want to try the tannin thing, without almond leaves, you can put in a little decaf green tea, or dry, clean oak leaves. They have the same effect. Indian almond leaves do sell on ebay for like, a bunch for a dollar also.
Also if you get a moss ball you should rinse it under the tap every week or so, because they tend to travel around the tank and pick up debris, they're almost like living filters :D
What ever you decorate with, make sure there's no holes that your betta can't fit through easily but could get stuck in, many have drowned because of this
also no really sharp edges on stuff :3