If you really want to buy a Halloween Head for 100m (I wouldn't recommend it) then you should do this:
Under "Game Content" on the top of the website you should click "Marketplace"
Once you're in the "Marketplace" go to the purple heading and click "Find Items" in the blue font.
Once you click "Find Items" you should see 2 options one that says "Part Type" and the other says "Alternative Search" click on "Alternative Search".
In "Alternative Search" search for "Halloween" and there should be a ton of Halloween parts listed.
Click the part that says "Halloween Head Surge" which I believe is the 4 icon on the top row.
Click the Halloween Head again and then press "buy".