Yes you can take your chances on driving in and hopefully finding a spot doesn't happen too often nowadays. you really need to call ahead of time in the further ahead of time the better.
Normally it is wiser to make reservations ahead of time to make sure those spots/days are available. Also if you have heard of dry falls, which is up the road from sunlakes, the visitor center is closed for the season, but you can still look out over dry falls.
It would be wise to reserve spots. Due to the popularity of this place, it can fill up fast. However, if you find a spot that's open you can pay on site.
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