Here's a couple of drivers I found by googling 2 x 3.5 speaker. (based on your pic, it looks like the standard 2" x 3.5" size)
http://www.ebay.com/itm/Speaker-2-x-3-5-4-ohm-3-watt-/280293284484
https://www.amazon.com/Aluminum-Rectangular-Internal-Speaker-118mmx58mm/dp/B00BQH5LMS/ref=pd_sbs_147_t_2?_encoding=UTF8&psc=1&refRID=F3VFB7YSCGHF2C5M3057
As clarkjd mentioned, that particular part inside the speaker is actually called a driver and the specs have to be as close as possible for best results. The one on ebay is 4 ohms, 3 watts. The one on Amazon is 8 ohms, 10 watts. Try to find markings on your driver and google the numbers/codes to see if you can get specs. Maybe the speaker cabinet or amp cabinet has a label or you can just...