To buy
Biking around the city is very convenient and also very enjoyable when the weather allows it. Finding and buying a bike isn't difficult and various options are also available.
First, you can simply go to one of the bikes stores that you can find all around Rotterdam. The quality of the bike will be ideal, but the price might be expensive; therefore using Internet and resell websites such as Marktplaats or Marketplace on Facebook could be a cheaper option and will still enable you to find bikes in very good state.
To rent
As you probably know, cycling is very popular in the Netherlands. Usually cycling is the fastest way to go from A to B in the city centre of Rotterdam. Most students will buy a second hand bike or get a monthly subscription to Swapfiets. Swapfiets will provide you with a brand new bike and your subscription will cover repair costs or provide you with a new bike when necessary.