Turtlebot 3 Burger robot, TurtleBot 3 Burger [US] - ROBOTIS, is a great way to get into robotics and ROS. Great documentation.
Not sure of the size, type, and price range you seek, but I have found Servo City, https://www.servocity.com/. to stand behind their kits and parts. They have always answered my emails and phone calls, as well as provided good advice and parts suggestions.
I’ve had good luck purchasing through Robot Shop and Pololu. Pololu has always provided detailed explanations and help with their hardware. Pololu forums are answered by employees and users.