235/40R18 and 255/40R18 will do fine. 235 will sit comfortably on the 8.5" and 255 will have the slightest stretch that you may not even notice. will be much cheaper in those sizes too since they are more common in 18" tyres. a 285/30R18 in the rear would be a bit of over kill plus heaps of money. a 30 profile is pretty thin also so ride will be hard and are more prone to damaging the rim due to hardly having a sidewall (pot holes, rocks, turning hard).
if you dont mind stretch on the back you can even do 235/40R18 all round as i'm doing on my 17x9.5 rim i have 235/45R17 and they are stretched but not too much. with a 40 profile it'll be a bit more stretched but should be fine.
the places you called must have idiots working if they have no idea on what fits those sizes.