I've been looking for Mirelurks for ages. So far, no joy. Now, there seems to be some movement!
A couple of pics for Mirelurk Hunters.

http://www.manticgames.com/mantic-shop/kings-of-war/trident-realm-of-neritica/product/trident-realm-of-neritica-mega-force.htmlThese will be coming as a regiment on their own soon. Unfortunately, they will be metal - I was hoping for plastic, as that would make them nice and convertible.
More pics can be see here:
https://www.facebook.com/manticgames/photos/?tab=album&album_id=10154829975777319Along with some odd octopus headed guys that might make for some fun 'Far Harbour' or ocean themed ghouls!
Then, there is this from Otherworld Miniatures:
http://otherworldminiatures.co.uk/shop/ua-series-underwater-adventures/ua2-giant-crayfish/They have a crab in the same series, but it is just a crab - and doesn't look like a mirelurk. They have giant scorpions, spiders etc in theit Wilderness encounters series as well.
For other mirelurks, I think there may be conversion potential in the Mantic Nameless:
http://www.manticgames.com/mantic-shop/dreadball/teams/product/kalimarin-ancients-nameless-team-14-figures.htmlKitbashing the lower half of the top four models with the top half of the bottom four miniatures looks like it may have potential as a base for FO4 Mirelurks.
However, check out this concept art for a miniature scheduled for possible release this year, as part of the Nameless faction for Warpath/Deadzone:

They are currently part of their Star Saga kickstarter, so the release may not be until the end of the year. But, should they come in plastic, they would be great as FO3 Mirelurks!