Possibly the Wrecking ball from the new W40k Ork Truk? (optionally with some of the spikes cut off)
http://www.bitsandkits.co.uk/shop....WT29%26I doubt I have a Custodian bell spare but I'll have a look in my big bag of titan bits on the off-chance I have a second one.
Bits-wise the bell from the Vampire Count's Corpse Cart could be good for the Custodian Bell? (with or without a little greenstuff to fill in the couple of little dents)
http://www.bitzbox.co.uk/product_info.p ... ts_id=3380Not sure how big it is, but I have one of these on order myself from the other side of the Atlantic so could let you know in a while. If it was too bit you could try Flagellant or Plague Monk bells.
Sadly neither of my above suggestions are in stock at those bits store, but they should re-stock at some point. Or you could try ebay, the seller 'Horde O Bits' has a few hundred auctions for current plastic bits and you could probably get them from him. He is a US seller unfortunately so would have to pay accordingly (the UK not having a similarly prolific seller to get bits from is annoying).
Best of luck with the conversion! Bits options are in many ways limited without being able to order metal bits, but having recently discovered the possibility of buying individual plastic bits has broadened my conversion possibilities a lot e.g. I was very happy recently to get all 6 of the certain mage's staff I needed from the plastic High Elf Dragon set to make some Exodite Support Chariots with suitably weird looking alien weapons
