I took voice lessons in my 40's after about 30 years of gigging. After a few lessons to learn some basic technique I have improved my tone, slightly, improved my range by a few semi tones, improved my diction.
I have always been considered a decent "rock & roll singer (in the Springstein/Cocker - rough voice style) by my peers and I traditinally sing about 10 songs per night at gigs (I don't think my style is good for more than that and I normally work in groups with other, better singers). But I was recording a CD and I wanted my vocals to be stronger. Even with lots of experiance and resonably decent vocal chops - lessons made a marked improvement.
To find a voice teacher, check your local music stores or music conservatory. You may find an add in the yellow pages. Talk to any singers you think are good and ask if they studied with anyone.
I find that with any instruction, you must first have an idea what you are looking to achieve (improved tone, higher range, improved falsetto, etc.) and clearly indicate your goals to the instructor. A teacher that wants to turn a rock & roll singer into an opera singer may not be a good fit.
As an example, many years ago, I wanted some piano lessons, to learn some basic theory (vs. playing Mary Had A Little Lamb). Some teachers had a specific teaching method (likely that is how they were taught) which insisted that I learn certain songs and play certain scales (all good things - if that is what you need). I did not need that - I needed to understand the circle of 5ths, etc. It took a while to find a teacher willing to teach me what I wanted to learn rather than what they wanted to teach.
