I'm trying to build a Resistance Thermometer that varies current/voltage output when heated. It has to be DYI, I don't want to buy anything done, only something like a light bulb, and this has to be easily changed, as the ideal temperature range I want to provide to it is 0-100 oC. I understand all resistors change their resistance with temperature, but I'm looking for something that changes it a lot more than the regular hardware store resistor.
You are looking for the temperature coefficient of resistivity. Iron is pretty high, I just looked up 0.0147 at 500C. Copper, in my table, is 0.0042
I use iron wire for a high current load resistor, and it is annoying how sensitive it is to heating.
Here is a link explaining it all.
You might be able to make a homemade thermocouple. If the range you really want to measure is 0-1000C, that's about the only way to do it. This is really more of a materials science question.
It's not obvious where you draw the line on "buy anything done". Even if you buy a piece of iron wire, it's been mined, refined, alloyed, drawn, heat-treated, etc. That's a lot of "done".
If the parts I linked don't quite meet your needs, be aware that there are several types of thermistors. Some are designed to have a linear response for sensing purposes like yours. But others are designed to have a strongly nonlinear response for particular control purposes (like over-temperature protection or inrush-current limiting).
And of course there are both negative tempco (NTC) and positive tempco (PTC) thermistors. You could use either one with some adapation of your software or other circuits.
Also I found that many of the linear thermistors available have a maximum operating temperature of 60 C or so, but the ones I linked is spec'ed for 125 and 175 C.