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it depends on how they have it blocked, but usually no, unless you can figure out the IP address of the site, then you might try and just go there, if they just have the domain blocked...it could work
 
If you had access to a proxy, you could do it. The iTunes Store uses the proxy settings in the network preferences. Whether or not the network is configured to allow any traffic out so you could actually use an external proxy is another matter.
 
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