What do you have. I'm no expert but I know about shaving a second or two off lap times. or checkout the tuning subforum. We have Praiano's tunes, MadFinn Tuners and quite a few more. Also,(shameless self promotion) We have the wateringhole which is my own stable . We do however expect a review for tune. I.e. you try a tune we have premade give us a review and we'll custom tailor a car for you, atleast most garages do this, but dont rule out using a premade tune if you find one(although a review would still be nice). Welcome to
you could go to the GT5 Tuning Forum. There are some people there who set up garages and do tuning requests. All you have to do is test their cars and post a review about the car in their garage thread. Do that and they will tune your car for you.
Here are 2 tuning garages that I can recommend for you. I can vouch for them 👍👍
I don't know why people think someone else can tune a car for them, nobody knows your driving style but you. Learn to drive the car to its limits stock then tune to fit you. You are the only one that can do that.