Can't help with an "over-the-counter" mitre gauge with a T-slot in the size you mentioned, but if you have a mitre gauge you may want to consider modifying it to fit as follows:-
Acquire a piece of aluminium (1/4" thick if that would fit in the track without protruding above) and have it cut/milled to a width of an inch and a quarter (or whatever width is the narrower part of the "T" track) so that it fits snugly in the track. It doesn't have to be of a "T" section --- all it needs to be is the correct width to fit in the track without any side play. Depending on the type of mitre gauge you have (presuming that you DO have one) the bar would probably need drilled in 2 places - one for the pivot point and one for the screw that secures the upper part to the bar. That 2nd hole would have to be tapped with the appropriate thread for the screw.
This wouldn't spoil the gauge from its' original use -- just make sure that you keep the original bar safely in case you want to revert the gauge back to its' original state.
The bar (the inch and a quarter one) could be made from hardwood instead of alloy/dural but it wouldn't be as stable and it would definitely wear quicker.
I know this could/should/would work because I did the same proceedure a few years ago in order for a non-standard gauge to fit my saw's mitre gauge slot.
