Nice alum ones on ebay for ~$20, if you don't mind Made In China. People say it is normal for the water to be 2" below the cap, and you need some air reservoir at the top. I used to see that in my minivans, and kept filling to the top, then noticed the level in the coolant reservoir kept climbing. The car just spews the excess out when it heats up, to get the 2" air gap it wants.
To verify your engine is truly over-heating, buy an IR Gun for $25 (sale at H.F.). I found mine to be amazingly accurate. Shoot next to the temperature sensor. I also use it to verify thermostat opening in a pot of hot water. You can also verify the sensor's resistance vs temp and your gage's readings vs test resistors. I have done so in several cars, and it is a bit of a science project, but I am geared up for such. A poor-boy approach is to watch if it controls to the same position on the dash gage, regardless of outside temperature. If so, the T-stat is probably regulating (thus not wide-open), so it just becomes a question of whether you T-stat is working correctly (see pot of water test).
