Late August as long as its not Shootout week (26-30th) should be easy to find rentals.
Look for condo's in Osage Beach. Most have boat slips or there are many resorts as well.

Generally the peeps in the area will throw maps at you with tons of local info. Also boat ramps in the area can enlighten your day. You will be fine!.

You guys are the besticles! Im waiting to see when my wifes audit for work starts. As soon as I can lock that down Ill make some reservations.