I am making a vinegar that calls for a sprig of lavender. I can't find fresh lavender anywhere but I can find flowers. How many flowers to a sprig?

Lavender can be a very strong taste and is also lightweight. One sprig can weigh under a gram. Dried flower buds of it weigh about 40 gram for a cup.

If for eating, start very small (like 1 or 2 buds), let the vinegar mature, then taste. If you want more lavender flavor, add more flower buds and let them sit about a week, and taste again.

Lavender vinegar can be intended for cleaning, or for eating (salads, desserts).
If for cleaning, you could use more...maybe start with a teaspoon of dried buds