Once you've finished the Chasing Echoes quest you'll be given another opportunity to become a Vampire Lord, even if you originally sided with the Dawnguard.
Details that may be considered plot spoilers:
Before you can enter the Soul Cairn, Serana will tell you that you need to be "dead." At this point, you can choose to let her turn you into a Vampire Lord or let her partially soul trap you. Using the former will (obviously) turn you into a Vampire Lord. In addition, you can choose to ask her to transform you at any point after this quest, regardless of what you choose for accessing the Soul Cairn. However, it's also worth noting that there is a dialogue that will give you the option to convince Serana to cure her vampirism. If you do so, you will lose the ability to have her transform you at later points since she is of course no longer a vampire herself.
I'll also note that this was how I played it, and I hadn't finished some of the Dawnguard sidequests before becoming a Vampire Lord, causing them to become blocked unless I cured myself (so be wary of that).