I urgently need answers; I can't really wait much longer!

[Zobia]

Salam Alaikum,

Islamically we're not allowed to consume bank interest and I know why but what if you need to purchase a product and the money you use to buy that might have been money that was in a bank account that may have consumed interest?
Thing is, I need a new pair of retainers; I really need a replacement and I'm not sure whether or not I can use the money I have to buy them because although it's not my money, it's my parents' and my aunty's money put together, the bank account where this money might have come from may have been a bank account that consumed interest, I don't know for sure but usually all the banks add on that little bit of annoying interest.. so I don't know if I can buy what I need.
And also, my family is going through one of the hardest financial situations; it's not terrible but it's hard. Alhamdulillah for how well it's going though, and I know that I am not able to get money from anywhere else, for a start I have an empty bank account; completely empty. It's been a while since I've had a bank balance. Getting more money would take me forever so I guess that is out of the question. So is it haram and forbidden for me to use this money that may have come from a bank account that consumed interest? (I'm not certain if it did), but am I islamically allowed to use this money?
And i have no other kind of money. Please support your answer with authentic ayahs from the quran if they apply and also support your answer with authentic hadiths as I need to know the right opinion; in fact all of us do

Jazakallah Khairan

[Zobia]

http://www.islamicboard.com/advice-s...y-worried.html

I'm stuck. Will someone with knowledge come forward and advise me inshallah?

:(!