What is the best way to travel from Koper to Porec? Our plan was to take a bus, but I am worried about the availability of buses on Sunday, which is the day we will be traveling. We will be dropping off a rental car in Koper and then want to get to the closest town in Croatia to rent another one, which I think will be Porec. If there is a better option, we would be happy to hear it.
The website Rome2Rio is a good one to know about for questions such as this, though it is not a total solution. Don't expect the trip-duration and fare estimates to be precise. Just keep drilling down for bus information until you find the name of the bus company(ies) and, with luck, a link to the schedule(s). If no luck there, Googling something like "bus Koper to Porec" will often bear fruit--though frequently you'll end up with the schedule for the trip in reverse.
In any case, you are absolutely right to be concerned about Sunday, which can be a tough day to get from one small town to another by bus in many countries. In this case, Rome2Rio lists four bus companies on the Koper-Porec route, though one has only two trips a week and is unlikely to be useful. Trips may be seasonal, so I hope you are not traveling too early in the year.
Just from looking at the map, I suspect you are right to head for Porec, a very pretty town that gets enough tourist traffic that I would expect it to have a car-rental agency. Other likely possibilities in Croatian Istria are Rovinj, Pula, and possibly the less-touristy administrative center of Pazin. All those alternatives are farther from Koper and appear to involved substantially longer bus rides.