What component should I use to build my own servos?
What sort of throw do you need.  There are, in fact, linear servos for model aircraft (e.g., http://www.spektrumrc.com/Products/Default.aspx?ProdId=SPMSA2030L) that might serve off the shelf.  You just send these guys a PWM signal-- 50% duty would be center, and changing the duty cycle would move in either direction.

My experience is that the servos are pretty well tuned off the shelf, as they tend to be geared down enough that the servo barely sees the load, so they work just fine for a whole range of loads simply by tweaking duty cycle.  You don't need to build a whole PID around them.