I recently let Yoshimo and Jan go, and I have need of them again. However, Yoshimo is nowhere to be found at the Copper Coronet and Jan isn't at his house or in the Government district.

Any thoughts on whether I can find them again?

When you let them go there's an option of leaving them exactly where you left them. So try to find them where you kicked them out. I heard that depending on the Chapter you are Yoshimo won't appear...you'll know why, trust me. So...I hope someone smarter and more experienced than me can help you. :(
Good luck! :D
By the way...Welcome to the boards!!!!! ;)