You could run into problems with two male dogs that are not neutered. They tend to be territorial so just having another male dog near your mom's dog could create a marking issue and possibly even some other behavioral problems. Of course a female in heat would be a horrible issue in many ways for both of them.
Yes, the better lines of dogs do often have waiting lists but sometimes these people can recommend someone else they know that breeds their lines but are much less well known. Contacting a local clubs could get you some good recommendations.
It is best to visit the breeder and also ask for references from other buyers, breeders and vets. Try to learn what a good pedigree would look like. Small dogs should be raised in the home and well socialized. There should be an official Yorkshire Terrier Club in your country, hopefully with a website. Contacting them should give you a wealth of information also. Avoid a breeder that has multiple breeds they are working with or anyone with over crowded or dirty conditions.
Ask the breeder what testing they are doing to make sure their dogs are free of genetic problems as well as disease. Gemy started a great thread with a lot of this information in it:
http://www.yorkietalk.com/forums/gen...s-yorkies.html