I have a gas cooker with two rings on it, one of the knobs is completely broken and I have to keep changing one knob to the other if using both rings at the same time.

Is there any place where I can buy a similar fitting knob?

Contact the manufacturer and ask them directly.

Many small stores that sell gas stoves also sell parts. Most cities or towns have a couple places like that. Not sure where you are but if you take the good knob with you they should be able to match it. If not contact the mfr. 

Any chance of posting a picture of your knob so we have a idea of what we're dealing with..

Universal replacement knob ...

Have you tried Lazada , these things are probably a standard size I would imagine.

 

21 minutes ago, Andrew Dwyer said:

Have you tried Lazada , these things are probably a standard size I would imagine.

 

The OP's knob is probably smaller and far from standard size, some things you just know.

Off topic troll post removed