You can trade, sell art, or code signatures. You can also try to enter raffles or watch the pound closely, although this make take a lot more time and leaves a lot up to chance.
Trading can be hard because everyone values pets differently. There are some good guides on the
CS Discussions board to help guide you on trade values. Don't be too worried about sending a trade which gets cancelled. It happens all the time. Just read the person's rules and try to be as fair as you know how to be. Until you have a good feel for the trading system I would suggest avoiding people who speak harshly (ie if they write something like "if you don't include my code word I will block you") in their trade rules as they are more likely to make an issue out of it. c:
If you have a talent for art you can try taking commissions
here. Some for coding, although you should make your topic for that
here.
Free Adoption topics and raffels can help build out your collection so that you can trade up more easily.
I wish you luck ^^