The Book Answer:
The Scrum Guide says that the scrum team has all skills necessary to deliver a potentially-shippable increment of the product. This is specifically put into place to avoid situations like you are describing. This is a classic case of "this is why Scrum says not to do this."
There are two ways to solve this problem and be within the scrum guidelines:
Get content members on the team. You mentioned that those people keep changing. Do they have to? Occassional change is normal but constant change is often a challenge imposed arbitrarily with no value being added from it.
Build the skills into the team. Often times we silo skills into jobs titles that don't need to be. Are you doing something so specialized with content that no one who isn't an expert in that area can create the content?
Pragmatic Answer
Sometimes you just can't - for many reasons including organizational constraints outside of your control or sometimes the skills needed are just too many and varied (for example, high end video game development often hits dozens of very specialized skills) and maybe you just have to have a content team and a development team. You can still fall back to traditional project management techniques of aligning priorities. How are priorities set with each team? Can they be set by the same PO? Is there an SLA from content that can be planned around?
Also, what is the value needed from your business? Are they ok with lorem ipsum and placeholder images? Does that give you enough to learn from your stakeholders and continue building out your next product increment. It's true that in Scrum we want to get to a completely shippable product every few weeks, but where are you now? If you're currently on a 6-mo release cycle and you're trying to get it down to monthly, maybe a partial fix to this is the big step forward you need right now.