what transformer should i use?
Before anyone can make a useful suggestion, we need some numbers:  What voltage and current do you expect from the Peltier device?  What power (volts times amps) is that?

Since the Peltier device produces DC, a simple tranformer won't work, as transformers require AC to work.

You will need a DC-DC converter to change the Peltier output to the 5 Volts you want to charge the phone.  5 volts at 250 mA is 1.25 watts, so the Peltier device must supply at least this much power (the DC-DC converter will be less than 100% efficient, so you'll probably need 1.5 - 2 watts from the Peltier device...)