You are right to consider this in advance. There is no magic solution. If you go to church and they find out your family structure you will likely be asked to leave. How will your extended family take it? Now, before you have a prospect in mind, might be a good time to start talking to them about it and how they would feel about it.
Do you have children and how old? Are they old enough to know what you would be doing? If so you might start talking to them about it in general terms.
What about schooling? Do they go to school? Are you prepared to home school if you need to?
Most polygamist do, and providing that support is a major mission of Biblical Families.
Where would your husband look for a wife if he were single? It is not that different. It could be single ladies that you know, perhaps in your church (before you get asked to leave). You could use a dating site. You could use a standard dating site, but then you have the added burden of explaining your alternative relationship/family. Or you can use a polygamy dating site, which in my opinion are not that good (and also in some cases may be shutting down to a bad new law). In my opinion that while the polygamy dating sites are excellent for entertainment value (like for playing "spot the fakes!"), they are poor at finding a new wife. I have only found one person that turned out to be an actual prospect on them.
Like a man looking for a first wife may take many years, many couples go a long time without finding a new wife.
Just as an example, the recent TV show "Seeking Sister Wife" profiled three couples looking for a new wife. There were no successes.