Maybe you can find a tank size you like in your local pet store, or shop online for good deals and put together a "Betta kit." (total cost of items+fish) Seeing as you are finishing another year of school with good grades maybe it could be for a "graduation present." If you chose a 10 gallon tank, and your parents would go for that size tank, it would open up the opportunity to divide it, and have additional Bettas. (The 10 gallon tank size is good for a lot of different uses, and pets, but the 2.5-5 gallon are great for Bettas too.)
Good luck with your parents though. :)