...I've bought the bulk Jagwire plastic lined stuff (about their medium grade) housing in bulk boxes from a couple of suppliers who list on Amazon, as well as some bulk boxes of stainless die drawn slick cables. They all seem pretty competitive in price, and it's hard to have a bad experience on Amazon buying something like this, because the bulk bike cables and housing are not something a lot of places care to carry and sell.

I did have one experience where I ordered the better grade stuff, and the supplier sent me the cheapest housing from Jagwire, but it was quickly remedied when I e-mailed them and explained the issue.

Niagara used to be good for stuff like this, but they're gone now. If you're not an actual bike shop, this is about the best you're gonna be able to do buying this stuff.