If they've passed their main board then it's a white stripe, making them appear not dissimilar to a pasted up fig 11. As to where to get them, I think a we knocked some up ages ago using spare slides and some white silk/cotton/anything to hand and they just get recycled. Perhaps your brigade RTC or whover handles modules 2/3/5 in your area can help.
Ask the Army School of Education (ASE) at Worthy Down. They run the Potential Officer Development Course (PODC) within which all POs, whatever their rank are designated as OCdts for the entire course (it was a 14 week course but I'm not sure now).
PODs (Potential Officers, ie: soldiers wanting a commission) wore blue OCdt slides and PRAMS (Pre-Sandhurst types, normally civilians) wore red OCdt slides.
The question of TAPO rank slides has been sent up the chain to RMAS. We suggested a DPM slide with a single OCDT stripe and the name of the RTC. This is in the style of the OTC slides, but probably in gold as the OTC ones all seem to be in black.
Aiming at the bottom centre of the white patch, go on.
I was on an exercise near the Iraqi border as an OCdt in 2001. Felt grrrreat plodding around with that handy identifier protecting my innards. Nothing happened, of course. I was just being a big scaredy-cat.