You're not the first person to end up with a cage that isn't big enough, including myself in that, and you certainly won't be the last! Pet shops are probably about 10 years behind when it comes to welfare standards so most people get caught in this same dilemma.
There are some good options out there tho and you've come to a great place to get advice. The best options differ from the US/UK/Europe but we'll get you and Creampuff sorted