Just head to your local scrappy with a flat screwdriver and a 6 mm torx key, hunt for a Scenic or a Megane (same speakers).
To see if the speakers are actually there, feel with your hand on the left side through the stowaway hole under it.
5 minute job, and you're done with a fiver.
Just got a few bits for mine today from another similar Megane, but with just 35000 miles on the clock:
You can buy aftermarket speakers that are made specifically for our car, but don't expect them to be cheap.
If you don't mind some electric DIY, you sure can buy some generic speakers... but you have to get them of the right diameter, shape, and not too deep, cut the proprietary connectors off your car wiring, crimp some fast-on on the wires, and plug them correctly (not inverting the wires, you'll get a funny sound if you do).
Not a simple task. I am an electronic engineer, so I am more than capable to do a state of the art job.
Don't bother, just get a pair of speakers off a scrapyard car as ours and you're done. Right shape, size, connectors... Eeeeeeeeasy. :d