I bought the 9th seed slot from Crazy Dave's shop, and now it says sold out. Is that it, or can I unlock a 10th seed slot from somewhere?
As per the Plants vs zombies wiki:
Extra Seed Slot ten - $80,000 (unavailable on iOS, Mobiles with Samsung bada OS, DS, and Android because the screen is not big enough to fit it)
If we think about the size of the screen, this may also be why it is unavailable on the Windows phone version.