At the end of an email to a stranger:
Glad to find you!
Glad to found you!
Which one is grammatically correct? Find or found?
is incorrect in any case. The first may be correct, depending on what you're trying to say.
is something you might say at the beginning of a letter or meeting. It implies that you have just found the person and are just starting the relationship. It sounds like you're leading up to something.
On the other hand,
is something you might say any time; either at the beginning or the end of the letter; it can mean the same as the above, or it can imply that you've already received a benefit and are saying "thanks, goodbye."
So, for example:
You technically have two different verbs in your examples:
As it happens, the past tense of to find also is found, and the source perhaps of some confusion here. When you put the "to" in front of "found" that way, it becomes the infinitive form of the second definition, which you almost certainly do not mean here.
The version you are looking for, assuming you have been searching for someone is the first
But more frequently you may hear either of these variations, which are slightly more idiomatic
In both cases, you use the first verb, to find but in the past tense.
You might use the second verb, to found, when talking about creating something. Such as in the following sentence:
Hope that helps