I want to run a character that uses Power Armor and Laser weapons, but in order to use the weapons, I need Fusion Cells. The only way I know of to get a supply of Fusion Cells reliably is to run the second mission for the BoS and the second mission for the Railroad, as both of these have you fighting Synths. However, I can only run these missions once, and I'm not sure I can get to the Railroad early, given it's location. Does anyone know of a way to get Fusion Cells in decent quantities early in the game so I can start using Laser Weapons as soon as possible? Preferably something that isn't just a one-off?

Maybe ask in "hint" section? In my game gangs of synths are everywhere.

You may find them on traders in moderate amounts and you can investigate the Merchant options in Settlements if you invest enough caps and charisma for the relevant perks.

Diamond city vendors tend to get a decent stack often, so does the wandering lady vendor that sell guns, a good early game loot spot is super duper Mart in Lexington, there are two minutemen corpses with ammo nearby.

Also remember to always loot the guns, guns themselves are a source of ammo.

I thought I had plenty of fusion cells, but found out that 2000+ won't actually last that long if it's for your primary weapon. If you're not also using melee or switching between guns with different types of ammo, then you'll need to buy it, and you might want to take the perk that let's you find more ammo in loot, too.

Stop using automatics.

I have a sniper laser rifle and my plasmas set up that way as well. You can fire as fast as you can press the button and there is significantly more damage per shot. 600 rounds lasts me a very long time on Very Hard.

Believe it or not, I don't.

First character I went sniper rifle + shotgun for close combat, and had serious ammo issues for both, eventually retiring the shotgun to replenish shells.

Second character is a melee build that started using Righteous Authority (critical damage bonus, specced for range, accuracy, damage). Now this character is running dangerously low on fusion cells. I have 2 Luck (few criticals) and I may take on fights too early, but it took ~50 shots, mostly hits, to snipe a super mutant brute. This type of battle doesn't happen often, but I've still used around 2000 fusion cells.

I actually love that ammo is scarce in the game, I just don't see how you can focus on just one type of gun without buying ammo.

It doesn't help that some of the energy weapons fire more than one cell at a time...

How long does a fusion core last on a laser gatling gun? Also with how rare plasma cartridges are early on I definitily go for a plasma sniper rifle or use it very sparingly

They're rare through out the game, but incredibly rare early on.

@OP, don't forget to pick up all laser weaponry you come across, even if you're just going to drop it back down again. When you pick up any weapon, its clip empties into your inventory.