There is a plate that screws into the bottom of my seat. Its fastens the seat to the post. I cracked it in half. How do I fix or replace it?! 
You mean you broke the seatpost? i dont really understand what you broke…sorry. a pic would help or a better description.
i think he broke the top of the seat post in half
if that is in fact the case, you just buy a new one. You can get them from UDC or wherever
-Pic
After looking closer I realized that your right! It is the top of my seat post. Here’s a pic. What I should do to fix this?
You could weld it back together, but that might be hard. I say just buy a new cromo seatpost. You shouldn’t be able to break that very easily.
buy a new one, welding would be annoying and it wouldnt be as strong next time, they dont cost too much. Also, make sure you by a CrMo one, much stronger than aluminum
looks like you are out a few bucks:(
How did you break it???
Highly doubt he has access to a TIG welder.
About a year ago myself and a friend whom both have KH20" unis broke our seatposts in that very same spot on the same day (literally about 10 minutes apart). The temporal proximity was especially surprising considering that I purchased my unicycle many months before my friend. The break also resulted in this picture:
Another friend bought the KH20 a bit later and had his seat post break similarly but seconds later the plate broke in the back too and the seat came off entirely. The whole scene was quite amusing.
That friend contacted Kris Holm directly who did not offer any sort of replacement/upgrade even considering the obvious design flaw. He did thank my friend for the feedback and said he would take it into consideration with future models of the KH20 but apparently that was not true.
We all ended up buying a chromo seatpost which we could only find on the UK’s unicycle.com site which resulted in a fairly high shipping cost. Those seatposts have held true for all of us though.
I also recommend removing assembly-instruction-stickers from your unicycle because they make it look like an item you picked up off the shelf at Walmart.
Well now i know what will be the next thing i replace on mine 
Can I get a CrMo seat post anywhere else other than the UK? Obviously trying to cut down on some shipping costs.
I was out doing some riding, I went to hop a snow drift and the unicycle didnt come up with me so I wiped fairly bad. I thought my post was simply lose until I examined it closer.
yeah, the kh20ers have an aluminum seatpost (if i remember correctly), and they tend to break really easily. So just buy a CrMo one and you will be fine
that depends. if you know what you are doing it isn’t that hard and you can even make it stronger than before. a little gusset and it is way stronger than before
really? ok… i’ll have to remember that