Has anyone ever booked a river and ocean cruise on the same trip? I am exploring this option and feel like I may need a travel agent to make this happen. Are there any good resources on this or advice? I don't know where to start.

I don’t think you need a travel agent. You probably do need to become familiar with some available cruises and have a good idea of what you like. You can see nearly every ocean cruise available on Vacationstogo.com (with approximate costs, cabins, ship plans, itineraries, etc.) and every river cruise on Rivercruise.com.
For reviews of ships and itineraries, look at cruisecritic.com.

It seems pretty easy to me. Pick a river cruise you like and an ocean cruise you like that begins a few days after the river cruise ends or ends a few days before the river cruise begins. You can fly or train between.
To get more specific advice, it would help to know how much total time you will have for the trip, why you are looking at cruises and any specific places you hope to see. When do you want to make this trip? Also, have you have cruised before and done land travel in Europe?

Ask moderators to place your post on the cruise form.

Most transatlantic cruises (ecxept Cunard Lines) start in early spring (repositioning) from Miami or so and are dirty cheap. They usually end somewhere in Southern Europe: Lisboa, Barcelona, Marseille, Savona, Civitavecchia.......
The most popular European river cruises start at Porto, Lyon, Basel, Strasbourg, Amsterdam, Passau, Potsdam, Cologne. They are much more expensive (per day) than the repositioning cruises.
Between these ports, you will have to fly (except for short legs like Lisboa - Porto, Marseille - Lyon, Marseille - Strasbourg or Civitavecchia - Basel).

Your post is tagged Hungary, Italy, Spain. Are those the specific countries you are interested in? Were you thinking a transAtlantic to or from Europe, or a European ocean cruise as well as river cruise?

From what I understand repositioning cruises are no longer as cheap as they were some years back. But I don’t have any personal experience, this is just what I hear and read.

There are quite a few river cruises on the Danube, starting/ending in Budapest, perhaps the OP means one of these as the thread is tagged for Hungary. There are Rhine river cruises too which go through Germany, but I don’t know of any in Italy.

As others say you don’t need a travel agent to book these cruises. Just look for a suitable crossing to Europe and a river cruise a few days/week after arriving your European port. You will need to take a train or flight to the river port and arrange hotels, etc for the interim period. All pretty simple to DIY.