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)
Walmart
I beleive AirsoftMegastore recently got them in stock.
A knife, field wound kit, .45 Colt Pistol, two extra clips, and the rest depends on what planes they fly.
Currently (April 2011) you do not have to pay extra.
extra cost all Delta forces guns are really bad
You simply have to reach the appropriate ranks. The Elite armor does not cost any extra cR.
Unfortunately you cannot purchase more clips for your Photon Burst unless you buy a new blaster.
Because they were considered Elite soldiers.
Umarex USA makes the H&K pistol. I suggest you contact them. See the link below
The Xbox Elite is only worth the money if you need the accessories and want have a complete system out of the box.
All the extra doo-dads do not make the pistol perform any better. Now if you want it to look cool and the extra $80 is no problem, then it will buy you a pistol that shoots the same with of without all the attachments. It's up to you and your wallet.
You can find the elite treadmill at Sears, Kmart, or at sites online such as Nordic track the can be delivered to your home. For an extra charge they will set it up for you.