There should be a button where you can download the asset pack after you bought it. If you missed it, from now on there should also be a download button at the top of the asset pack page that you've bought. If you can't find that, then click on the down arrow icon at the top-right corner of the screen, next to your username. A dropdown menu will appear. Click on "My Library", and you'll be taken to a page with all the things you own on itch. You should be able to find it there.