Yes, the best place to buy them is off players - or at the Grand Exchange.
Otherwise the most easiest, but longest way is to pick-pocket h.a.m members to get the clothe parts.
Members; G.E.
you can get them from HAM members
You can go to the grand exchange and buy one there, or go to Thessalia's Fine Clothes shop in Varrock and buy one.
Yes, they are members' items.
pick pocket ham people.
Yes, but the hard way - you have to re-do the events.
In varrock they let you change your clothes at the hair cuttery.
the grand exchange on runescape
You can buy membership at stores like Target or at the Runescape website.
Only buy runescape membership from their site, look on homepage.
nothing
There are no Grim Reapers clothes in RuneScape. There is just 'Shade Robes' which look very similar. You can buy shade robes from other players, via either the grand exchange or by trading.