Where might I find a 3mm slot cutting bit

I’ve searched most of my usual router bit & end mill places, but I haven’t been able to find exactly what I need… a 3mm (or 1/8") tall slot cutting bit, 1/4" or 1’8" shank with a outside diameter of around 5/8" to 3/4". Even a 4mm (or 5/32") tall slotting bit would work
I’ve got several 1/8" tall slot cutters already but they’re over 1" in diameter & they don’t fit in the areas where I need these little slots machined.
I’ve been thinking about grinding some T-slot router bits down to 3mm or 4mm height, but i’m not sure how well they’d hold up over time.
Material cutting is polycarbonate (lexan).
Any ideas or good places to buy from?

You don’t mention the depth of the slot cut but whiteside has this:

Or possibly a Dremel #199 slot cutting bit…

that whiteside is just what i’ve been looking for, only need about 1/8" to 3/16" of cutting depth.
i tried the dremel 199 with a dremel, but i’m not sure how it would hold up in the cnc.