Where do you plan to be? I travel without advance reservations except for my first and last stops. (There are over 20 stops in between.) I make arrangements, mostly using booking.com, via the Wi-Fi in my hotel room, most often 1 to 4 days in advance. The timing has been gradually increasing, because I've found myself stuck with places that were perfectly fine but either not as well-located as I would like or a bit more expensive than I prefer. And, as noted above, it does take time to do the online searching. I would never want to just show up in an area and look for a place for that very evening. The world has changed since the 1970s, pre-internet. Everybody except you will have pre-booked. And you know those folks chose the best places.
In general, July and August are difficult in coastal towns. You'll be competing with, it sometimes seems, half the population of northern Europe as well as many folks from the country you're visiting. Having a car is definitely helpful, since you're not tied to choosing a place in a high-demand coastal town. Look (online, in advance) for lodgings at inland locations instead.
One place where I had a great deal of difficulty finding a place on shortish notice was in Croatia: Rovinj, on the Istrian Peninsula. I planned a fairly short visit there, then I learned of a one-day bus tour to some inland towns I wanted to see. The bus tour only ran on Wednesdays, so I needed two or three extra nights in Rovinj, starting the next day. All the reasonable places were booked up, so I started visiting the little storefront travel agencies, each of which seemed to have its own notebook of private rooms on offer. Eventually I found a place that was a bit farther from the historic area than I wanted to be, but still walkable. It was a very nice room with its own kitchenette, in a home in a rather new neighborhood. But the male half of the couple who owned the house was a chain smoker. I got to see Motovun and Groznjan, but I would not have chosen to stay there if I had known.