All the black hash you can find in thamel is laced.
Some pollen is pure, but very very far from the real deal.
You will get an ounce for 30$, You can get 500g for this price if you're smart.
Get the hell out of Kathmandu, at around 4 hours of bus you can reach Malekajung, before Daman (Pollen/Hash), or Maleku(on the way to Pokhara- Buds), where you will get decent deals even if you do not speak nepali. Just ask around somebody will help you at some point.
Yes you can buy small quantities, but considering the price, and that travelling around with it is pretty safe, I ll reckon you get enough of it. For example 10$ will give you 100g of herb if you bargain. That is the price. November will be a nice seasons to score some of the best buds. Ask for purple haze and silver haze in Maleku, that how they call their tops buds there.
Trust me if you follow my instructions you will find what you are looking for...
Then go travelling around and you may be able to pick up nice small deals, but even this get difficult these days, especially if you don't know much about hash, most of it is laced, even in the mountains...
On the way to annapurna, you will see ganja everywhere, so north of pokhara. Way to daman as well, you'll get use to spot them after a little time
They are all over