I am not sure this answers directly the question, but it can help. Actually, I think you asked two questions.
1. How to get hold of the icons?
a. Google Directions API question has very useful info.
b. This answer to Google Maps v3 Direction Icons is probably the most useful.
c. This answer to Get between streets in Google Maps.
d. This is also interesting.
2. Where are they stored?
The location you are looking for is apparently not yet documented.
See for instance this.
One place to get them is tt3.png (similar to what you found).