It's not easy to get your hands on spare parts for these cheaper prams. You should be contacting a seller of this brand in your country and asking if he could order the part you need (in your case, the wheel). Another way to try is to write to the manufacturer, the Riko brand, directly: try this customer service contact form. I can't promise you they'll send you one, but it's worth a try.
There is, however, the possibility it's not the wheel that's broken, but the internal securing latch doesn't hold it in place - and that won't be fixed by a new wheel even if you got it somehow. Try consulting any stroller repair service (many shops have their own service technicians, try to ask around) for advice as to what can be done.