Well, big dogs arent allowed in HDB as well. =\ you can go to the ava website to check out which dog breeds are approved.
of cos, i would recommend you guys to adopt rather than to buy.
it's giving these dogs a second chance.
one negative point is just that most of them arent puppies. but like seriously, it's still a dog afterall. it would still grow up in the end right? (:
you can try your luck at SPCA although most of the dogs there are not HDB approved. there are other animal shelters such as mutts and mittens and animal lovers league. you can go google them to find out more.

there are also dogs seeking for loving homes online. mostly by their foster parents. you can go forum hunting and who knows? you might find the furkid you want. <3
lastly, when you go to view the dog, please bring your whole family staying with you along. this is to see if the dog and your family can get along and if your whole family likes the dog.
to end off, GOOD LUCK! ^_^
Suzume