Does anyone know where in the U.S. or Canada I can buy extra long kit-kat bars for making eg. barrel cakes  (like the pig in mud cake)

I don't think there are extra-long kit kat bars.  They are all about 3.5"-4" long.  Even the king sized bars are just pretty much a twin pack.  I could be wrong, but I've never seen a longer kit kat.

I did see a recipe with a picture where it calls for giant sized kit kat bars.  I haven't seen them myself either.  Hopefully somebody might have an idea.  Thanks for the reply.

They don't make them extra long. The "king" kit kats just give you an extra row, but they're not longer. For the ever more popular pig hot tub cake, that is a single layer cake...I can tell because the kit kats are taller than the cake.

Today I found the chunky kit kats in a longer bar but I think they are too "thick".  There is now a video on youtube of someone making this cake (it is actually a double layer cake not single layer in all the recipes) but she uses the regular length kit-kats and it works fine.  Thank you to those that replied.  Your help was much appreciated.

I checked my usual UK chocolate site and I don't see any extra long ones on there, or in Canada, so I guess they don't come in extra long.