What can I do about yellow teeth?

I'm too young to whiten my teeth by the dentist (plus my parents won't agree). I want to buy the Crest Whitestrips but there are none for sale in Thailand and it is very expensive. Whitening kits aren't an option either because I can't go buy it without my parents knowing. I tried using baking soda for a while now but I see no differences. Please help!