Hi Alastair Welcome to the forum.
With MOCs, it really depends on a lot things- How much money you wanna spend, How fussy you are with condition and which card releases you're into: Star Wars, ESB, or ROTJ or Tri-logo. Generally they go up in price from former to latter, tho that's not always the case.
Is there a particular figure or card-back you had in mind? Lobot??
Let peeps know what you're after and they might be able to steer you in the right direction.
There are a few for sale in the sale thread on here. Bollux, Stargeezer both have a couple going and Spoons has a couple of 'Han Hoths'.
Check em out.
Of course there's always ebay. If you see something on there and are unsure, just post a link here (ebay thread) for advice.
I have a ROTJ Rancor Keeper and Tri-logo Nikto going too :wink: ... but they're not posted yet.
Cheers
Pete.