You can add another one at any time unless you feel your dog does not get along with other dogs. I would start socializing him with other animals and see how it goes. I brought Gabby into the pack after my 2 babies were already two years old and although they wanted no part of her at the begining they now get along just fine.
There are plenty of Yorkie rescues. I would start searching the net and finding a rescue that are fostered in your area or near. Sometimes you have to travel to pick up your dog. You can go to
Petfinder.com: Adopt a pet and help an animal shelter rescue a puppy or kitten. and type in the info with your zip and anything close to you will show up.
Most rescues require that you are approved before adopting therefore I recommend you start now since most require you to fill out a application and do a home visit. This will make it easier for you so that when you find the one you want to adopt you're already approved. Please know that there are alot of people who want to adopt younger ones so don't get discouraged. Just get the approval part of it out the way. They require a adoption fee which most think its high but every dog is spayed/neutured, have dentals, vaccinated...Good Luck!