Hi,
welcome on the forum !
There is not many 700c production unicycle but you may get lucky on one shop doing a custom build.
Your best bet would be to build your own. You start with your favorite 700c rim and tire. Just make sure your rim has 36 holes (most common on unicycle hubs) or at worst 32 holes.
And from there, you get it built on a unicycle hub and you buy the rest of the part on UDC for example.
You can have a look at the Building a true 700c uni.
Have fun =)