I'm trying to make a headlamp that's powered by the heat of your forehead and don't know where to buy a step-up transfomer. Do any of you know any good websites or store I can go to?

 

A step-up transformer is not what you need for this application. You will be using either thermocouples or Peltier devices to produce electrical power from the heat available, and both of those devices produce direct current. The light source will be LEDs , and they use direct current. A transformer is used for ALTERNATING current, not part of this discussion.
The show-stopping problem that your idea will run into is that there is not enough heat energy available, even from a hot-headed person, to light an LED to any useful brightness.
I suggest studying thermocouples in an engineering level text that will provide actual numbers in units of actual power.

 

What transformer? The one in this picture? 1:100

Coilcraft www.coilcraft.com
LPR6235-752SML (1:100 Ratio) LPR6235-253PML (1:20 Ratio) LPR6235-123QML (1:50 Ratio)
Würth www.we-online
74488540070 (1:100 Ratio) 74488540120 (1:50 Ratio) 74488540250 (1:20 Ratio)

You might be underestimating the power in a forehead.

 

LTC3108 is an ultralow voltage booster.......... " the LTC3108-1 is a highly integrated DC / DC converter ideal for harvesting and managing excess power from extremely low voltage input sources .the step-up topology operates from input voltages as low as 20mV. Using a small step-up transformer, the LTC3108-1 offers a complete power management solution for wireless sensor networks.operates from Inputs of 20mV"

That's amazingly low input .... output adjustable 2 to 5 Volts ...It may just be possible to power a very small led ...

The LTC3108 seems to come complete you just put your peltier as input and an led on output ...I would try wiring it like this and experiment ...

I had one of these peltier units at hand and wired it to a voltmeter , with body heat I could just get 100mV

 

I see part numbers listed in post #3, and if you have part numbers and the money you can try Digikey. They probably have them in stock, or will have them shortly.
But consider that while LEDs are rather efficient, they seldom generate power, and so to get a useful amount of light takes a lot more power than to operate a micropower data logger device. So just because you can have voltage does not mean that you can have enough power for the application.

And it is true that SOME LEDs will generate a voltage when exposed to bright light. Not all of them, but some certainly do generate voltage.