filament wise: pet-g, polyflex, ninjaflex, flexismart etc.
Just search for: flexible filaments on amazon :-)

Well for $500 bucks probably the best thing you can buy is this: http://wanhaousa.com/products/duplic...us-steel-frame

if you want a kit, you can probably get a larger build volume: http://www.ebay.com/itm/New-Tevo-Bla...kAAOSwdzVXvVMF

For $500 those are probably your best bets :-)