I bought used speed and RPM sensor by echowell. Here is a picture with this product and I highlitghted in red components that are missing. So as I unserstood this is magnet for sensors. Question: can I buy regular magnet for sensor in bike store or does this system require specific magnet made by echowell?
To a large extent a magnet is a magnet (so long as it's strong enough). You may have to mount it in the correct orientation. A spoke-mounted magnet is quite easy to make or cheap to buy. The one that mounts on the crank for the cadence sensor may be a little harder, but I've got one that just relies on fixing to the pedal axle with its own magnetism (from a sensor that's too cheap to actually work reliably). You might not need a bike shop in that case.
It will be pretty unlikely that a bike store will carry just the magnets. There's simply not enough demand for them.
However, it's very easy to find these replacement magnets through the internet. Just checking Amazon showed me several. I'm sure any of them will work: