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The Surge wasn't designed to have a replaceable strap so fitbit don't sell replacements. If you're comfortable with DIY then you can find replacement kits on the likes of eBay but the process is not straightforward (there are videos on YouTube to help).
Alternatively, if you are still within warranty then you may be eligible for a replacement Surge. If you contact customer support then they can check your warranty status and advise on your options: contact.fitbit.com