Remember the adage, "you get what you pay for." But then again, you are wise to look for the bargain. Cashmere is expensive for a reason. If you buy cheap fiber, you'll be very frustrated and have worthless yarn.
However, many people have recommended this eBay seller: colourmartuk. I've never purchased from them, but they have excellent ratings and you can see pictures of what customers have made. They also give detail about their yarns in the listings, so it tells me they know what they are selling. They even have a free sample pack, so you can try samples for yourself - very cool. I keep telling myself to get one, but haven't yet. They don't seem to have anything larger than aran.
Then there's School Products. I haven't ordered from them yet either, but they have a good rep. They have some bulky cashmereright now.
Not to mention some cash/merino on conesor in skeinsthat comes in lots of colors. They have Karabella cash too, but more $$.
That's as much as I know (not much!) and those are the 2 places I would consider.