If you want a numnah shaped one then it's tricky, numed do them that are cut to fit (come to think of it I have a black numnah for mine I need to sell ...)
I tend to just use white squares, again I have a numed with half fleece which fits perfectly, but have also invested in a couple from 'horses in sports' - they attend the BE stuff and their white squares for £15 are bloody brilliant!
The squares the are resonably forward cut tend to fit well although I do tend to by a size bigger some of the time as cob size seems to fit the saddle better than pony size. Most of the time though I use a fantastic sheepskin jumping saddle shaped numnah that I bought in France for £7 as a lot of the saddle cloths over there appear to be better suited to the K2 than the ones over here.
I use my black sheepskin numnah under mine at the moment which fits it perfectly but i see what you mean about finding saddle cloths that don't look too big as the flaps are so short & forward cut. Ive had my new K2 a few weeks now & im so pleased with it. One of the comfiest saddles ive ever sat in!!
I love my K2 Jumping saddle, so comfortable! I use a polypad with mine atm which is fine. The only other numnah that fits is a william funnell one - but again a bit bulky. Its a shame to put a bulky numnah under a well fitting saddle tho