Overchange billing

I was moved from nashua mobile to altech autopage last year, the first few months i noticed that the billing was high and i ignored this, until i took a closer look at the bill and realised that my contract had been changed from the 1g I used to receive to 100MB, I called to have this resolved and i was told that i need the original contract from nashua mobile. how and where am I supposed to get this??