Somthing like this?
You can also look at other pattern companies on line - Butterick, McCalls, Vogue come to mind.
Do you have a local Joann Fabrics or other fabric store? You can look in the pattern books there.
I'm not sure what you mean about the seat belt holes, though. Wouldn't the seat belts go on the outside of the clothes anyway? Oh, I guess you mean for between the legs? You could measure up approximately how long the baby's legs are and put a hole/slot where ever you want it.