Eric Talbot does commissions. You can contact him by email with
[email protected]. I am not sure the charge rates he has since I have not bought art or prints from him like I have Dan Berger and Mike Dooney.
http://erictalbot.blogspot.com/
Peter Laird does not do commissions only turtle heads with his signatures. You can contact him via his twin PALogs off of BlogSpot or send something to Mirage Studios preferably with a return address envelop and postage for easy return.
Kevin Eastman is only really obtainable via conventions. He does it a lot more last year and this year. He is a great guy and is cheap. I met him in Charlotte last year. Hope that you are near one of them. His official website I see is hyperlinked above and I suggest you take a look at it. You can buy pre-signed comics or a t-shirt but not sketches.
Sorry, if its not much help but its quite hard to get these guys to do what you are asking.