Advice on replacing 12 in woofer in Martin Logan Monolith III's

Unfortunately when I got these they did not come with the passive xover.

If I can't find ACI-SV12's,  what else should I look for?

I've emiled ML to find out the cubic size,  but who know if they will respond. 

I've seen some people use the Peerless SLS 830669. Thoughts?

George do think the ScanSpeak Discovery 30W/4558T would be my best option?  I dug around looking for some SV12's, no luck yet.

Thanks for all the input.  I have reached out to ScanPeak  for more data on the Discovery 30/4558.  Also inbox the guy that had the SV12's for sale,  but haven't heard anything. Even though the post was a year ago, I think maybe he still has them.   I will be putting a sub with the setup,  so I don't need it to down real deep.  Question:  what would be the ideal  crossover point for the ELS  running at three-way?

That's good news!   Hopefully he will respond to my inbox.  Thanks for all your help George!

Thanks for the info Tim.   Finding that driver is the problem  :).  It seems they are very hard to come by so I'm trying to figure out what the next best thing is. 

I will still look for the SV12's but if I can't find them what driver would you guys recommend?  I'm just not as knowledgeable on driver specs.  Being that I 'm running a SVS sub,  I need a driver that is a good mid-bass.

So the box is not square as George indicated, it is 25 inches wide and tapers to 19 inches at rear.  24 inches H and 11 inches deep

 Correction on the box size it goes back 6 inches before it starts to taper

You would think this  measurement would be on the Internet. I've looked at MartinLogan site and around the net, but can't find it. 

With a lot of advice and help from Tim (Timlub) I purchased a Dayton 12 inch driver. Since I'm running a dbx 260 active xover, I was able to to dial in the xover points and the slope,  it sounds amazing.  Also I added a SVS sub, which really brought out the lows.  Still have some room EQ I need to deal with, but overall it sounds awesome. Thanks Tim!