Arnor figures from the Games Workshop LOTR skirmish game ("Middle Earth Strategy Battle Game") are good, but very hard to find and not in sufficient variety to build an army.
Vendel Miniatures has a very nice range that is appropriate for Arnor and its successor Kingdoms (Arthedain, Cardolan, and Rhudaur). Note that Rhudaur had the least surviving Numenorean influence, and the strongest Hillman influence, so figures for that army are discussed elsewhere.
If you're looking at historical figures for substitutions, anything appropriate for the Byzantine Dark Ages (7th through 9th centuries) will do very well.
Hillmen would represent a significant proportion of Cardolan and some of Arthedain, especially the levy foot. They are best represented as more rustic and tribal forces with lots of fur and leather. Slavs, for example.