There are LOTS of great small breeders around, it may be easier to ask around for what you are specifically looking for. Most breeders sell their joeys around 8 to 10 weeks, it's actually quite difficult to find an adult to breed. There are a few different schools of thought on keeping them from breeding too young, and you won't get many people to agree on just one. You can either get 2 young ones and keep them separate until the female is more mature (8 months or so), start with a younger female and wait until she is older to add a younger male, or you can put them together early and attempt to separate them during heat cycles. The last option obviously comes with the greatest risk of early pregnancy but allows the joeys to grow up together and hopefully establish a greater bond before becoming parents.