Where purchrase cheap MTB frame?
As the question is formulated now, there is no definitive answer. It is really up for you to decide. Here are some hints though.
You can either buy online, or in person. In both cases you can buy either new or used.
When buying online, you cannot &quot;feel&quot; an object before getting it, compared to buying in person. However, in both cases it is possible to see obvious problems if you are careful enough and you ask questions. For used frames that would be crash damage. For new frames it would be manufacturing/QA issues, such as poor welds.
You would not be able to tell &quot;hidden&quot; problems in either case, such as internal cracks or poor welds hidden behind coating, so there is always risk of getting something unusable. For used frames, there is rarely a possibility to claim warranty. For new frames, it depends, and usually the law is on your side as long as you have proof of purchase.
Remember that a bike frame is not a small object in its dimensions. When buying it from overseas or even ordering the delivery by post, freight costs are likely to eat a considerable part of your budget. When buying in person, this is something that can be avoided. But when buying in person, there is a small chance of ending up robbed and getting nothing (again, I do not know where you live and how bad the situation is there)

I need a frame with disk brake mounts so it's hard to find a cheap bike just for the frame

Buying a bike just for the frame is almost always unwise when money is considered.