Mmm... Use a 7555 - works with a supply 2...18 volts. I'll find a circuit, it's a real beginner thing...
Here
http://www.intersil.com/content/dam/...icm7555-56.pdf
Use the circuit in figure 5. Use equations 1,2,3 to find the resistor and capacitor values. The output from pin 3 should drive your LED with a resistor in series, with the LED, possibly 220-470 ohms depending what voltage you're supplying.
For this to work properly the capacitor C should be ceramic, 0.1 microfarad, it won't work with electrolytics. RA and RB will be of the order of 470k - 4.7megohm.
BTW I saw a nice idea for a finder the other day - instead of using a red dot it had two pieces of Perspex, each with a small circle engraved about 1 cm diameter. The Perspex pieces were about 5cm apart, with circles aligned along to optical axis. Basically looking through these, the circles appear concentric when you are on-axis, alternatively if you are off axis one appears displaced accordingly. There was a red LED on the side which illuminates the circles using internal reflection within the Perspex. Would be a really easy project to make one like this !