I’ve installed an aftermarket cruise control on each of my last 2 cars; granted they were not as new as your 2006 model, 2000 Miata and a 2000 Honda Insight.
On the Miata I used an Audiovox model which I had zero problems with but I don’t believe they’re available anymore.
On the Insight I installed a Rostra unit which is a little nicer and has a wireless steering wheel mounted switch, it was also a little more expensive.
Both were fairly straightforward installation-wise although if you’re not comfortable with finding the necessary connection points in your cars wiring harness I would suggest finding a professional to do it for you. I’ve had no issues with either system and they have worked wonderfully.
You can find information and pricing on the Rostra system here: http://www.brandondist.com/products/custom_cruise.htm
I found my unit a little cheaper than what Brandon Dist. offers elsewhere but don’t remember where.
With the amount of driving I do cruise control has been well worth the effort and expense, when purchasing a used car you don’t always get the choice of factory options and in the case of the Insight it was never available anyway.