Hi Gopherboy,
Don't be surprised if Bunnings can't help - you may be better approaching a large format/industrial printer and enquire about the types of polycarbonate materials they have.
I went through a similar process when looking for a suitable material to use for our overlays and I ended up going to this company:
www.aawade.com.au . I'm not saying you should use them but at least it gives you another "avenue of attack".
When searching for Lexan I went thru the website mentioned by cdbrown and was contacted by a salesperson who then referred me to a company called Cadillac Plastics, who are apparently distributors for Lexan products. I don't have any contact details for them but AFAIK they are still around and would be worthwhile contacting.
As an aside to Lexan, Perspex or Lucite, have you considered having your artwork laminated then using an industrial type adhesive to fix it to your panel? Just another idea!
Feel free to drop me a line if you think I can be of any more help.
Cheers,
ChrisW