Some trees only become available for purchase at the store at certain levels:
Chicken - level 2
Cow - level 2
Piggy - level 12
Goat - level 19
Pony - level 26
Pearl Shell - level 30
Octopus - level 30
Sheep - level 34
Bunny - level 34
Ducky - level 40
Bear - level 47
Hedgehog - level 55
Cat - level 64
Moose - level 65
Giraffe - level 70
Peacock - level 75
Alpaca - level 80
Cute Panda - level 85
Arctic Fox- level 87
Elephant- level 90
Manis - level 100
Pink Flamingo - level 101
Turkey - level 107
Kangaroo - level 110
Camel - level 117
Turtle - level 123
Rhinoceros - level 126
Narwhal - level 130
Dolphin - level 133
Seal - level 145
Platypus - level 150
Lemur - level 152
Precious Lobster - level 160