How can I bleed my hydraulic disc brakes without buying a kit?
It depends on your brakes.  

Some models need nothing more than a couple of short lengths of plastic hose, a cloths peg and syringe. Others need fittings to attach the hose(s) to the brakes, and you may or may not find something else to do the job in a hardware shop.  

Kits are mostly relatively cheap and probably cost no more than individual bits purchased  from a shop. Some kits are expensive (Avid comes to mind) - probably because you have no choice and have to buy it for the proprietary bits needed to do the job.