More specifically, 18mm ones.

I have a pink(ish) perspex one with a 4.2mm hole. I have no idea if thats good, bad or indifferent!
A quick search has thrown up a myriad of options from £2.50 to £20 per element, pink, amber, centra, metal, glass, perspex all in a million different sizes.

Can anyone suggest a starter pack of type/size to suit informal target, plinking and occasional long range silliness?

Many thanks