Bike shops may have a bunch of left-over pads from helmets they've sold and fitted to the customer on site. Ask around and you should find something suitable.

BTW, helmets don't wear out or deteriorate unless they are badly abused or involved in a crash. Tests of 10+year old helmets show they still meet their original impact specifications. That said, newer helmets do fit better, are better ventilated and may meet tougher safety standards so buying a new one isn't just a waste of money.

Keep in mind that the cheapest and the most expensive helmets by any of the good makers all meet exactly the same safety standards. More money buys you better ventilation, sometimes, lighter weight and, obviously, more style points with your riding buddies.