I have files structured as follows:

```
folder1
-file1.a
-file2.b
-file3.c
folder2
-file1.a
-file2.b
folder3
-file1.a
-file2.b
-file3.c

```

I want to copy just the files with .a and preserving the tree structure:

```
folder1
-file1.a
folder2
-file2.a
folder3
-file3.a

```

How can I accomplish this using bash?