I keep expanding this list as I overcome challenges and discover workable substitutes.

If you can't find this You can use this
A
   
   
B
Baking powder Combine 1 tablespoon of bicarbonate of soda (baking soda) with 2 tablespoons of cream of tartar.
Buttermilk For 1 cup buttermilk :
In a wet measuring cup, add 1 tablespoon vinegar.
Then fill the cup with milk.
 
C
Cake flour For 1 cup cake flour :
In a dry measuring cup, add 2 tablespoons cornstarch.
Then fill the cup with flour.
Coriander root There is no real good substitution but coriander stem is probably the closest thing you can get.
Crème fraîche For 1 cup Crème fraîche :
1 cup sour cream or
1 cup (240 ml) whipping cream plus 1 tablespoon yogurt or
1 cup marscapone cheese
Cream of Tartar Equal amount of lemon juice or vinegar
(1/8 teaspoon per egg white). You may want to add a little sugar.
D
Duck sauce  
   
E
   
   
F
Fish sauce Equal amount of soy sauce
   
G
Galangal Substitute with ginger and lemon juice
Gelatin sheet (leaf) 5g gelatin sheet = 3g powdered gelatin
H
Hoisin sauce  
   
I
   
   
J
   
   
K
Kaffir lime leaves Equal amount of lime zest or lemon thyme
   
L
Lemongrass 1 stalk of lemongrass = 1/2 tablespoon lemon zest + dash of grated ginger
   
M
Mirin (Japanese sweet rice wine) 2 parts sake + 1 part sugar or
white wine + sugar
Molasses 1 cup Molasses = Substitute with the same amount of honey, dark corn syrup, maple syrup or 3/4 cup brown sugar dissolved in 1/4 cup liquid
N
   
   
O
Oyster sauce worcestershire + soy sauce
dark soy sauce + hoisin
   
P
Plum sauce (see duck sauce)  
   
Q
   
   
R
Ricotta Cheese Equal amount of cottage cheese
   
S
Self-Rising Flour For 1 cup self-rising flour :
1 cup all purpose flour plus 1 1/2 teaspoons baking powder plus 1/4 teaspoon salt
   
T
Tamarind paste Worcestershire or fresh lime juice
   
U
   
   
V
Vegetable shortning For 1 cup vegetable shotning :
1 cup unsalted butter or 1 cup lard
   
W
Wasabi Horseradish
Worcestershire Equal amount of soy sauce (add a small amount of red wine)
   
X
   
   
Y
   
   
Z