Disc brake pad replacement, or replace whole caliper?
If you cannot find replacement pads, many cheap off brand disk brakes use BB3 pads, have a look and see if they are the same. Changing the yourself is straight forward enough. Sometimes its as cheap and easier to replace the caliper, especially if paying a mechanic to source hard to find replacement pads. If you do, get one that is major brand, or at least takes major brand pads. 

If you want to do it yourself and cannot find pads, there are two mountings for calipers, IS and Post. Generally buying same style caliper as you already have makes it a simple replacement. Adapters will probably cost more than a cheap caliper. 

Disks come in two mounting standards as well. 6 bolt and center lock. If you want to replace the disk you will need the get the same type.