Backpacker - I'd probably ask your airfield what they mean by Hi-Viz jackets. If they need to meet a European Class then you're stuck with some sort of silly hat. However they may be happy with anything with a reflective stripe on them (lets ignore the fact you aren't doing it at night so they have no headlamps to reflect) so you can pick any colour you like (I'm impressed that some websites still call ones with black backgrounds
Hi-vis jackets 
)