You want an adapter from Modern Astronomy in the UK - that's the only place I've found that sells them:
Modern Astronomy - Accessories
In the drop down list, select the "T Thread Adapter for Straight Thru Adapters" under the "Adapters for SkyWatcher 50mm Finders" section near the top.
Your SSAG (identical to a QHY5 camera) has a T thread that the 1.25" nosepiece screws into - the nosepiece screws out easily.
I used to have such an adapter for my QHY5 but sold both a while ago.