I run an online store that sells Linear actuators and no 2 actuators
are exactly the same speed so we also want to start selling a small
low cost variable speed controller for people who use 2 actuator
together but need them to run the same speed. I think you electrical
guys call them Variable resistors or Rheostats. Our Actuator run on
12vdc and pull approx 350mA no load and approx 4A at full load. Can
anyone reccomend the ideal  part number and supplier for us for under
$10 each

A rheostat is one way to vary the speed of a DC motor but it is
considered a low end solution where the motor speed can easily wander.
 Pulse width modulation is oconsidered a more reliable way to
accomplish this but is far more expensive.  Using encoder feedback
with a "real" controller is far more reliable but of course is way
more.  Getting back to your original question, I don't believe anyone
makes speed controls at your desired price point, however an SCR used
for model trains probably comes the closest.  Newport Corp makes a
Model 861 open loop variable speed control but it costs $300.

There is a decent discussion on the subject of DC speed control you can find here:
http://www.lmphotonics.com/vsd/vsd_01.htm

Here is a set of plans from Minarik (44 pages):
http://www.minarikcorp.com/PDFs/250-0215.pdf

Big Blue