at an isuzu dealer or a honda dealer. you may not be able to get just the sending unit anymore. check both sources before giving up.
here are two
stcharles isuzu dealer in missouri sells parts on the phone, discount,freeshipping and expert advice from Merlin (and they are open sat. morning.
http://isuzu.stcharlesauto.com/Department-Parts.aspx
and here's a honda dealer with an online site
http://www.slhondaparts.com/
and here's the official isuzu online parts site, but you still have to contact a dealer for the parts (though the site has a way to email them directly from there.
http://www.myisuzuparts.com/
someplace there is a thread or two with parts numbers for just the sending unit, but i think that is old info
good luck
//bc