Retired Credit Shop pets are Omni, Liyure, and Kioka. You can only get them by buying them from users, since their eggs are no longer available in the credit shop. You can click search pets on the navigation bar to see what they go for in rancher shops. Aerix are available currently, and can be bought with credits in the credit shop. You can get credits by buying them at a rate of 10 cents per 1 credit, or 100k Tu per 1 credit. You can also do free offers that will give you some amount of credits. Aerix eggs are sold in threes and cost 200 credits.