Andrew Smith Smith3637@msn.com
A Realtor is probably the quickest and easiest way to get comps.
You could pay for an appraisal, but in many/most cases that's not necessary. It might be if you had a unique property and there weren't readily-available comps, but that doesn't sound like the case from your question.
Or you could do it yourself. I've done.
As for how recent the sales should be: The more recent, the better. In today's market, for most purposes, you'd like data from within the past 90 days. If that's not possible, go back further, to 180 days. Beyond that, the numbers are likely to get shaky. Then you have to weigh whether the best comp is in your own neighborhood, say, 210 days ago, or in a nearby neighborhood 30 days ago. There's no perfect answer; it's often a balance of proximity and time.
If, in your neighborhood, other people have successfully appealed their assessments, and you have a similar property, then their current assessment (the lower one, after appeal) does provide evidence of the value of your home. So, if you can find properties in your community comparable to yours with a new, fresh, lower tax assessment, then that data works in your favor.
Your local assessor's office probably has some detailed instructions on how to appeal an assessment. Follow those procedures to the letter.