I've been trying to find this information on the Web but to no avail. Read up that for rear wheel, the best size will be 17×6.25 inch. As I see alot of softail 17×6 inch wheels on ebay for cheap, I wonder if it will fit into my 2009 streetglide with minimal effort and cost. Thank you.

 

With some spacers a little time and effort you might, But.. you'll lose the Cush Drive sprocket assembly, Not worth it.
It would be a major PITA to get wheel centered, Sprocket and Brake rotor lined up.

 

Yes. I did it on my '09 and have been running a 17x6 softail wheel with 17-200 rear tire for over a year. Need a 68 tooth sportster pulley, custom 5/8" rotor spacer and custom axle spacers. Forgot measurements on axle spacers but they were only like a 1/8" different on each side. Works fine and I don't miss the Cush drive.