Looking for an extreme duty machete. Must be at least 14in. long by 1/4 in. thick. Can spend up to $300. Any help or advice would be appreciated.

I don't think this is the proper forum group for such a request. Unless you are looking for an antique or historical reproduction.

If that's the case, I'd suggest getting yourself a Billhook or a Grosse Messer instead.

You could try Ben Potter for a custom forged blade to your specifications, he does have an economy line that would be in your price range of $300 depending on design. ( Thinner stock )

http://www.seekyee.com/privateerarmoury/
http://www.seekyee.com/privateerarmoury/commission.htm

Although for a 1/4" thick blade the " QUARTERDECK LINE" starts at a bit more $500 to $1000 price range.

For factory blades Coldsteel has a large variety of types and styles of machetes but the stock thickness is closer to 2.75 mm

http://www.coldsteel.com/machetes.html

For thick bladed machetes you could look here: http://www.machetespecialists.com/mawithblforh.html
(I have no personal experience with this vendor).

Try Blades Forum: http://www.bladeforums.com - a search for machete finds 105 posts... or better still buy a decent billhook - for $300 I'd also want it gold plated.... $30 will buy a good one....

What do you mean by "heavy duty"?

A machete is designed to cut light vegetation, making it too thick would make it unsuitable for that task...