I would like to become better and stronger at spotting combinations. I can't buy any chess books at the moment. What's a good website for learning combinations?
Chesstempo has a tactics trainer with solving against the clock and a tactics rating. I think there are a bunch of other sites out there with similar features.
If you are rather old school WTHarvey has 10,000 chess puzzles, just as a collection of diagrams with mouse over solutions.
Chessgym also has tactics against the clock and some other features like playing endgame positions against tablebases.
You certainly do not need a book in order to learn combinations.
Chess.com has in my opinion the best tactics online. Unfortunately, you only get 5/day if you have a free account, so it's not the best. Personally, I work on other websites with free tactics for a while, and then finish up by trying my best on these 5.
Chesstempo has a lot of good tactics too. And they're all free.
Lichess.org also has a lot of tactics under their Training section. It has good puzzles related to the coordinates of the board and/or opening theory as well. Currently their tactics seem to be more checkmate-focused than other websites, but I've seen this site improve constantly for a year now and every aspect of it (including the tactics section) has truly been getting better and better every week.
I encourage practicing tactics puzzles whenever you have free time. You don't need to go to a website on the Internet; you can also use apps on your Android phone or Tablet. TacticTrainer has a large problem set, and you'll get presented with problems that are generally rated within 200 ELO of your level. You can specify a starting level, but the app keeps track of progress, and as you get better, gives you harder problems to solve.
I also use Chess King apps from ChessOK; Combinations I and II are good, but cost a few dollars.