Construction grade lumber has a greater number of growth rings per inch.
In the big box stores and most lumber yards, all 2 x somethings are construction grade. Its just a matter of sales volume. Lets face it, 2 x 6 x length is used 99.99 percent of the time for construction applications.
In rare cases, finishing lumber in a 2 x something can be found in a Woodworker's Specialty Stores. Be ready to pay near three times the cost of a big box store's construction grade 2 x something.
The big box stores will have 'finishing lumber,' which is wide grain (few growth rings per inch). In this group a 1 x something is fairly common. Thicker cuts will get you back to the specialty stores.
If you are willing to take the time to sort though the shipping stack of lumber, it is possible to find 'lighter' than normal 2 x somethings in a stack. Note that the store staff will be weary of what you are doing, but will generally not halt you: If you stack the lumber back properly!
Here your goal would be to find the 'lighter' 2 x somethings in the stack. The lighter boards will be somewhat softer than standard. But, note that the vast majority of 2 x somethings will be standard or heavier. After all their use is for building something!
If you visit your local big box store during off hours, commonly evenings for the lumber section (contractors are not in the store). You can ask the staff if they (or other customers) have pulled any lighter than normal 2 x somethings from shipping stacks. Tell them why you're looking for this type of lumber. Remember, this stuff will be rare, maybe a couple per shipping stack.