I've been using this for 3 years on pots up to 40 gal

http://www.northerntool.com/shop/tools/product_200586484_200586484    $80

Also, the old N Tools tree pot mover link doesn't work.  Here's the new link.

http://www.northerntool.com/shop/tools/product_200631819_200631819  $200


The link to the bigger plate dolly above doesn't work for me because of extra, invisible characters after the .html.  Here's a link that does

http://www.nurseryenterprises.com/large-pot-dolly.html     $460

We're not told the widest pot this can handle but given the 16" wheels it looks like a 20" diameter pot will fit but a 24" pot might not.  So this might handle a 20 gal pot but not a 30 gal pot.  For the money I'll stick with the $79 NT hand truck.  Even if it wears out 3 times sooner you'll still save money buying 3 of them.