Barry,
Both are herbs. The lemon balm should be fairly readily available anywhere that you normally buy fresh herbs. I know that you can grow lemon thyme but I'm not sure how easy it'll be to find. I'm sure if you wanted to you can just add lemon zest and possibly a good lemon oil or pure lemon extract if nothing else.
Here's a recipe that I got from the Cooking with Caprial website recently.
Lemon Verbena Bread

2 1/3 cups flour
1 1/2 teaspoons baking powder
1/2 teaspoon salt
1 1/2 cups sugar
3/4 cup butter, softened
3 large eggs
1/2 cup buttermilk
1/2 cup sour cream
1 tablespoon lemon zest
2 teaspoons finely chopped lemon verbena (also an herb)
Glaze: 2 tablespoons sugar
juice of 1 lemon

Preheat oven to 350
In a large bowl, combine flour, baking powder, salt and sugar. Add softened butter and mix with a fork until flour mixture resembles coarse crumbs. In a medium-size bowl, mix together eggs, buttermilk, sour cream, lemon zest and lemon verbena. Add to flour mixture and mix just until batter comes together. Pour into a greased and floured 9 x 5 inch loaf pand and bake about 1 1/2 hours or until knife comes out clean. In a small bowl mix together the sugar and lemon juice. when the loaf is ready to come out of the oven prick the top with a fork. Brush the glaze on a little at a time as it cools. Use all of the glaze. Serve warm or cold. This loaf freezes very well. Just warm it a bit to bring out the lemon flavors.

If I don't find Verbena soon I'm just going to make the loaf with extra lemon zest. Let me know if you like it.
Pat