I've sided with Dawnguard, but I chose to become a Vampire Lord during Chasing Echoes. Now the people in Fort Dawnguard won't sell them to me. Is it possible to get bolts while being a Vampire Lord?
It is not possible to use any of the Dawnguard vendors as long as you are a vampire yourself. (They'll all point you at the Morthal quest to cure your vampirism).
The good news, is that once you finish the Dawnguard Main Quest, you can ask Serana to turn you into a vampire lord whenever you want!
So as long as you have a supply of black soul gems to return yourself to being a mere man (or mer), you can bounce back as much as you like between vampirism and normalcy (as the cure vampirism quest is, in fact, repeatable).
So to answer your question, no, but you can continue to be a vampire lord with a little extra effort on your part.
If you're playing on PC, you can also access the console to acquire bolts that way.
You can loot bolts off Dawnguard members, as you'll clash with them frequently if you side with Harkon.
Additionally, you can get Dwarven Bolts by going through the various Dwarven Ruins around the map. They are found on the Spheres and Ballistas.
You can also craft them at a forge with the smithing skill, steel and Dwarven as applicable, with some firewood and the relevant ingot. However you only have access to the crafting recipes if you've started Sorine's quest to fetch the improved schematics. There's a chance that you started this before Vamping out.
Additionally, you can pickpocket a crossbow and a single bolt onto a practicing guard, and take their bow. They'll then shoot an infinite number of bolts into the practice target. You can do this by giving the crossbow and single bolt to a follower and cleaning up after them.
I kill the dawn guard that aren’t particularly of a quest that’s a fast way to get bolts I get 1