Captchas are in their greatest abundance in profile comments, so all you have to do to get some captchas really easily is to click on the link to comment someone back.

Now right-click on the Captcha and select "Save Picture As" (IE) or "Save Image As" (Firefox) to download the image used. Upload the image into Picasa Web, Photobucket, Tinypic, or any other image hosting web site. Then, to post the image just copy the image URL and past it inside BBCode image tags.

You can hit the reload link above the captcha to go through a bunch of them without refreshing or navigating away from the page, so you can search for ones you think would make for some interesting mad libs.