Here is a website that seems to have all sizes of the original fusion boots in-stock:
Aqua Lung Drysuit Boots | DUI RockBoots | Scuba Center - Eagan and Minneapolis, Minnesota
www.scubacenter.com
The price ($135US) is a bit spendy but that is what the regular retail price was/is for these boots:
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The boots have a velcro inside that the velcro on the sock feet line up with and their is velcro on the rim of cuff....there is also a plastic piece that goes inside the sock/feet of the suit to help "lock" the boot and suit together.
I installed the boots on my suit and use one of those hangers that the ankles slide into to hand the suit upside down to dry.
Because the velcro inside the boots is the "hook" side, I put a strip of loop side velcro over the areas that the socks/feet didn't line up with to prevent unnecessary wear and tear of the fabric.
The boots stay attached so well that I have only pulled them off a couple of times since owning my suit (2018).
Just note that the Fusion boots don't offer much in the way of ankle support at all. I put insoles from my hiking boots in the socks above the flat plastic piece and that made them more comfortable from an arch support perspective but did not add ankle support.
If you need ankle support then the Evo 4 boots on the same website are a good recommendation.
-Z