Buying someone else's plans is an OK idea for someone that lacks imagination, inspiration, or can't think of ideas. It isn't very hard to draw boxes and designs on a piece of paper to show what you want your cage to look like. Then, if you can't build it yourself, you can always take the plans to a carpenter and have them build it for you.
All you do when you buy someone else's plans, is get a cage that the originator liked. If you are truly going to build a cage that you like for your animals, then design your own so that you can make it what YOU want it to be.