I will be traveling to Berlin and plan to rent a bike and or go on a bike tour of the city.
Are bike helmets available with rentals or should I plan to bring my own?
I would bring my own. The European are not as obsessed with wearing helmets as we are. We have rented bikes a number of times and do not remember seeing helmets. But since we had our own, we didn't ask either.
We brought our own helmets and seats and my wife brought pedals, but we were on two separate week long tours in Mecklenburg-Vorpommern area. Having your own helmet avoids concern about "ukus".
You may want to correspond with Berlin bike rental agencies to find out what is available: http://bicyclegermany.com/bicycle_rentals.html
If you don't have access to a helmet, make sure you have medevac coverage.