Where would be the best place to look for a Honda Insight?
Check craigslist, your local classifieds, cars.com, autotrader.com, etc. The Insight is no longer in production, so your only option is to buy used.
Does anyone have any thoughts about what the best year is?
Newer is better in terms of battery life. Also, the Insight with CVT transmission will have better battery life because it manages the battery pack better than the manual transmission.
Would it be better to buy used or new? and if I buy used what's an ok amount of previous miles for an Inisght?
Your only option is to buy used.
Is there any way to make the car even more fuel efficent?
The Insight is already designed for maximum efficiency. You need to learn how to properly drive it to maximize mileage. Read this article for more info. It's geared towards Civic Hybrid drivers, but the information will apply to the Insight as well.
http://www.cleanmpg.com/forums/showthread.php?t=1306
Good luck in your car search!