I am planning on running a 3" stretched swingarm on 06 xl, can I get some places to purchase belts that would know stock length to stretched length? Thanks...

 

You might be able to use a stock one from another bike. I think it's 1 full tooth for every 0.50" extra so you'd probably be looking for a belt that's bigger by 12 teeth.

Might want to confirm that lol.

 

You might also be able to go down in size on both sprockets to get some more length!

 

Try a John Deere dealer. I heard somewhere a few years ago they use the toothed belts on some of their equipment. Probably cheaper than Harley, too.
Let us know what you find.

 

Who's stretched swing arm is it??? Because most every manufacturer already has those things figured out before they sell them and know which belts fit properly. In which case you should ask them.
If it is going to be a home-made swing arm, then I would start with the belt's available, such as on Softails, and go from there. Design the lenght of the swingarm to match the belt you can use.