Where can I but the internal filter for the allson trany besides the dealer?

From any Allison dealer (there's a dealer locator on the Allison web page). The internal "filter" is just a pickup tube with a metal screen and not a filter at all. You should only need to change this if you have a known large item contamination issue (more than a strainer required then!) or if you are changing pans to an Allison deep pan or a Mag-Hytec (which also requires the deep pickup).