Finding a Wali as a Revert
Not having a Muslim father or male relative doesn't leave the wali requirement, where it applies, unresolved. Islamic scholarship has long addressed exactly this situation, and mosques with any revert community handle it as a normal part of arranging a marriage, not an exception.
Who can step into the role
Many scholars hold that a recognised imam, or a qualified representative of a mosque or Islamic centre, can act as wali when a Muslim woman doesn't have an available Muslim father or male relative. This is a well-established position rather than an improvised workaround, though how often it's actually used, and the exact process followed, varies between communities and schools of thought. See the role of the wali, explained for the general background on what a wali actually does and where scholars differ on the requirement itself.
What if your father is alive but not Muslim?
This comes up often among reverts, and there isn't one universal answer. Some scholarly views and local practices differ on whether a non-Muslim father can act as wali. This is a question to ask your specific mosque or Islamic centre directly, since the answer can depend on which school of thought your community follows and how marriages are registered where you live.
How to actually arrange it
Speak to the imam at a mosque or Islamic centre you're already connected to, ideally well before you need the arrangement finalised. Being upfront about your situation, rather than raising it at the last moment, gives them time to guide you through whatever their community's process actually involves.