What is the bar called inbetween a cast garden bench?!?

Threaded rod / threaded stud / threaded bar.

In your situation you may do best to buy a piece of steel rod from a metal supplier (yellow pages) - most towns have them - and have him cut a thread on each end. Much cheaper. Measure diameter and length - allow a bit for the pieces that stick out at the ends.

In the UK very probably sub £10...
From a bench seat manufacturer very probably £30 +

Depending on length needed DIY superstores also sell threaded rod but very often only in lengths up to a metre.

Peccavi
April 2011