With a low chance, random seeds will drop by picking up wild growth like colored grass and bamboo sprouts. What seeds drop when picking wild growth depends on the season, and they usually grow quickly to maturity, like Pink Turnip and Turnip.
Harvesting crops will yield seeds of that crop regularly, not for all though. Alternatively, using a sickle on a fully grown crop will provide higher quality seeds, though the crop will be lost and can no longer be harvested. This works even better on re-growing crops as the plant isn't damaged.
The following tables show a list of all seeds, how to acquire them, after what event they become available, their planting season, how long they take to grow, whether they regrow, how they look when finished and their cost & gain. Take note that purchase prices are from seeds and sell prices from their respective crop.
Spring Seeds[]
Plant
Lv 1-10 Buy
Lv 1 Sell
Lv 10 Sell
Days to Grow
Multi Harvest?
First Appearance
Where To Buy
Image
Pink Turnip Seed
80G
200G
240G
3 Days
No
Beginning
General Store
Cabbage Seed
100G
480G
960G
6 Days
No
Beginning
General Store
Strawberry Seed
260G
720G
864G
9 Days
2 Days
After 1st Dungeon
General Store
Onion Seeds
110G
780G
1,560G
12 Days
No
After 1st Dungeon
General Store
Pink Melon Seed
620G
4,200G
6,300G
7 Days
No
After 2nd Dungeon
General Store
Moondrop Seed
70G
600G
3,000G
7 Days
No
Beginning
Fantastic Flowers
Toyherb Seed
60G
180G
900G
2 Days
No
Beginning
Fantastic Flowers
Cherry Grass Seed
120G
860G
4,300G
6 Days
No
Beginning
Fantastic Flowers
Lamp Grass Seed
480G
4,580G
22,900G
18 Days
No
After 4th Dungeon
Fantastic Flowers
Emery Flower Seed
1,000G
12,800G
64,000G
27 Days
No
After 4th Dungeon
Fantastic Flowers
Blue Crystal Seed
700G
7,000G
35,000G
16 Days
No
After 4th Dungeon
Fantastic Flowers
Gold Cabbage Seed
230G
3,800G
11,400G
14 Days
No
Beginning (?)
Special, Yue (Random)
Summer Seeds[]
Plant
Lv 1-10 Buy
Lv 1 Sell
Lv 10 Sell
Days to Grow
Multi Harvest?
First Appearance
Where To Buy
Image
Cucumber Seed
80G
180G
216G
6 Days
2 Days
Beginning
General Store
Tomato Seed
140G
300G
600G
11 Days
1 Day
Beginning
General Store
Corn Seed
130G
440G
880G
13 Days
2 Days
After 1st Dungeon
General Store
Pumpkin Seed
220G
1,240G
2,480G
10 Days
No
After 1st Dungeon
General Store
Eggplant Seed
160G
480G
720G
9 Days
2 Days
After 2nd Dungeon
General Store
Pink Cat Seeds
50G
270G
1,350G
4 Days
No
Beginning
Fantastic Flowers
Fire Flower Seed
260G
1,200G
6,000G
6 Days
No
Beginning
Fantastic Flowers
Ironleaf Seed
420G
5,500G
27,500G
25 Days
No
After 4th Dungeon
Fantastic Flowers
Clover Seed
390G
2,100G
10,500G
10 Days
No
After 4th Dungeon
Fantastic Flowers
G. Crystal Seed
700G
7,000G
35,000G
16 Days
No
After 4th Dungeon
Fantastic Flowers
Gold Pumpkin Seed
3,300G
15 Days
No
Beginning (?)
Special, Yue (Random)
Autumn Seeds[]
Plant
Lv 1-10 Buy
Lv 1 Sell
Lv 10 Sell
Days to Grow
Multi Harvest?
First Appearance
Where To Buy
Image
Potato Seed
90G
280G
336G
3 Days
No
Beginning
General Store
Carrot Seed
100G
590G
885G
8 Days
No
Beginning
General Store
Yam Seed
80G
140G
210G
6 Days
1 Day
After 1st Dungeon
General Store
Spinach Seed
60G
140G
168G
2 Days
No
After 1st Dungeon
General Store
Green Pepper Seed
150G
480G
620G
9 Days
1 Day
After 2nd Dungeon
General Store
Charm Blue Seed
40G
320G
1,600G
6 Days
No
Beginning
Fantastic Flowers
Autumn Grass Seed
440G
5,400G
27,000G
15 Days
No
After 4th Dungeon
Fantastic Flowers
Pom-Pom G. Seed
510G
6,600G
33,000G
22 Days
No
After 4th Dungeon
Fantastic Flowers
Red Crystal Seed
700G
7,000G
35,000G
16 Days
No
After 4th Dungeon
Fantastic Flowers
Gold Potato Seed
250G
1,900G
6 Days
No
Beginning (?)
Special, Yue (Random)
Winter Seeds[]
Plant
Lv 1-10 Buy
Lv 1 Sell
Lv 10 Sell
Days to Grow
Multi Harvest?
First Appearance
Where To Buy
Image
Turnip Seed
130G
420G
630G
3 Days
No
Beginning
General Store
Leek Seed
120G
500G
750G
6 Days
No
Beginning
General Store
Daikon Seed
160G
770G
1,155G
7 Days
No
After 1st Dungeon
General Store
W. Cabbage Seed
200G
1,350G
2,700G
12 Days
No
After 1st Dungeon
General Store
Hot-Hot F. Seed
470G
4,800G
12,000G
18 Days
No
After 2nd Dungeon
General Store
Noel Grass Seed
330G
4,221G
21,105G
24 Days
No
After 4th Dungeon
Fantastic Flowers
W. Crystal Seed
700G
7,000G
35,000G
16 Days
No
After 4th Dungeon
Fantastic Flowers
Gold Turnip Seed
210G
1,400G
5 Days
No
Beginning (?)
Special, Yue (Random)
Tomato, Corn, and Pumpkin Seeds don't tell you how much they cost in the Diamond General Store.