You can get it in the grocery store. I've heard you can grind it up and put it into capsules and eat it, but I've never used ginger for nausea. I've always used benadryl or dramamine, both of which work nicely.
ive never experienced nausea during a trip on shrooms.. and ive only tripped with like 2 people who have ever had that prob. i think its just the person.. how many shrooms did you eat btw? if anything, try some Tums, or pepcid ac i think..? any medicine to settle stomachs would probably work.
You can get ginger as a vitamin at most places that sell them. Don't have any experience with them though.