Where can you buy extra clips for the Colt Delta Elite Air Soft Pistol?

You can't.

First, it uses a magazine, not a clip.

Second, if you are referring to the spring powered toy that retails for around $35 USD, the spare magazines are simply not available as individual components.

If you are really lucky, you might find someone on Craigslist or eBay that has a broken toy but functioning magazine that you can purchase from them.

Your best bet is to buy a spare or 3 for the additional magazines.

(See the link below)