Body armor for butt

Do you mean padding like on a Triple Eight Bumsaver short?   

Or do you mean actual hard-shell impact-spreading armor like is found on the Dainese Wave models (some of which extend to cover the sacral area)?

I highly reccommend Azz Padz

http://www.azzpadz.com/

We bought these for each of us after my wife broke her tailbone. Most body armour for your butt is just foam underwear but Azz Padz have a hard plastic form fitting shell sort of like an ergonometric avalanche shovel and on the inside it is lined with thin dense foam. It doesn't show (no panty lines) and doesn't affect mobility in any way. Ski poles or 2x4s won't make a dent. It has saved my own ass at least once - I woke up in Emergency with a severe concussion from a heelside slam on a snowboard. I was unconscious for 4 hours, my helmet was toast and I had a headache but my butt was top notch without even a bruise. 

This sounds like a post best suited for newschoolers....

There are all types and kinds of armor and there are several threads on it here. Dianese is awesome stuff but there are also crashpads, POC and many other brands. As said above, depends on what you want it for.

I don't have any direct experience, but I would check out the ice hockey section of the sports store as an option.  It's designed for athletics so should translate over to snowsports really well without a huge price premium for niche markets

I've had a few different kinds. I suggest going and trying on a bunch until you find one that fits and is comfortable. If it isn't comfy, or doesn't fit, you won't wear it, so it won't help at all.

I only wear padded shorts when I'm snowboarding on hard snow or racing.

Hockey pants. BTW--do hockey players still use garters to keep their socks up, like we did in the old days, or is there a better method now?