Thank you for considering a rescue yorkie for your Wrigley's sibling. Saving a yorkie life is one of the most rewarding experiences I've ever had.
As young as your boy is, he will adjust to a sibling now, easier than if you had another over a year or so old. Timmy had a great idea about starting now to socialize Wrigley.
You would be surprised, I think at the number of yorkies showing up in shelters these days. I work with another rescue locally that is an all breed rescue and the director tells me a year ago, they NEVER got yorkies. Within the past 5 months, she has now had 8 yorkies surrendered to her. Yorkies are bred so much now, it was inevitable the shelter numbers would rise
Just a month ago we acquired a 16 week old female, so you see even young ones are starting to show up!
I disagree about the the average rescue's fees being too high. There are cost involved as megansmomma mentioned. And, I've said before - I consider rescue a matter of the heart and not the wallet.
Here's a link to a Florida rescue:
Yorkie friends rescue