There is NO such animal as a "30 day visa on arrival" (as Happy As Larry pointed out those are SOLD to people from 19 countries, and they're only good for 14 days).

You also don't need to download a form to enter thailand on a 30 day visa exempt stamp.

This leads me to believe you're either a clue-less goof-ball, or you're wanting the form to apply for a 30 day extension of stay because you have a visa exempt or a tourist visa entry.

If it's the later and you actually want to apply for an extension, you need form TM.7 on the link that Fondles posted already.

Unfortunately if it's the first part of my previous statement I can't help you at all.