Most of them have websites/online shopping where you can easily check. I've never seen catering sized sliced pans in any of the main stores myself (Tesco, Dunnes, Lidl, Aldi, SuperValu).
You could ring one of the bakeries like Brennan's and ask them where their available to buy
They might even arrange them to be delivered to a local shop or if you're lucky they might deliver to your door
If it's something you want regularly (daily) ask your local convenience store if they can get it for you.
They are 1200g and slices are about 20% bigger than standard pans and a denser slice too.
Price back in 2015 was about €3.20
I think it is JMOB bakeries that do it. Edit - all the main bread bakers do them.