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.
Assuming it's the same as other Plants vs Zombies games, 9 is the maximum number of seed slots you can have.
You can get 10 seed slots on the Xbox 360.
You can get up to 10 slots on plants vs zombies. I personally wish there was more because I am gaining so much money so quickly.