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Marketplace is the most likely to come across a Proton Pack being sold, though completed ones don't pop up every day, or even every week some times. eBay is an option to consider, though if you do look there, it'd be in your best interest to check out this list of sellers (some of whom are very active on eBay) you should avoid, as they're proven to be bad news:
The Blacklisted Sellers List Something that would also help potential sellers gauge your commitment is if you posted a rough ballpark figure of how much you're looking to spend, as a quality Pack with Lights and sounds can easily go from anyway between $500 and $1,000 depending on the parts and craftmanship.