i have an iphone 3g with firmware 3.1.2 and jailbroken, and i want to restore the phone. The thing is, i dont have a restore file on my PC because the original restore file was on my macbook which i sold

So guys, is there a way i can restore it back to 3.1.2 or 3.1.3? I dont want to update to 4.x because of the speed issue

Can i just download the firmware from some website, and then restore to that file ?