hello!
Addons can be found either here: construct.net/en/make-games/addons
or here: construct.net/en/forum/construct-3/plugin-sdk-10
Why can't I find some addons on the 1st mentioned place?
(example: construct.net/en/forum/construct-3/plugin-sdk-10/effect-mode-ported-c2-140280
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It's entirely up to third-party developers where they publish their addons. Ask the developer if you think they should publish it to the Addon Exchange - that's what we designed to be the one place to find addons.