I started playing a new Fallout New Vegas game and decided to work only with energy weapons this time, but I have great difficulties finding enough energy cells for my plasma defender. I know you can converse MF cells into Energy Cells, but I have a hard way finding any types of cells. Gun Runners doesn't seem to stock any types of cells and the Van Graffs have only a small amount of energy cells available. What are my options?

If you help the BOS enough, they will give you energy weapons ammo in that box inside their bunker entrance.

Also OWB is a good source of energy ammo, of course you're stuck there until you finish the main quest.

IIRC the Legion might leave you a lot of energy ammo in those resupply boxes up in the rocks near Cottonwood Cove, if you have done a lot for them.

Otherwise, recycling the spent ammo will give you a little more.

Energy Ammo is rarer than normal stuff. The Scrounger Perk helps with this. Also, Robotics Expert to get more from them when you destroy them.

Hit up the Traveling Merchants and other minor merchants for this ammo. ED-E can make them for you once you upgrade him in LR, Muggy can make them for you each day. Vigilant Recycler helps.