I assume that you do not know much about Thailand at all. Also, your comment clearly tells you don´t give a <deleted> about your own well-being.
I happen to know, since renting many time before, that most Thais that rent bikes and also many foreigner just don´t care about the helmet and that rentals do not give them automatically with the bike. However, every time I asked to have a helmet, they always had a pile of them to hand out if somebody asked for it.
And, If they do not have, then YES! You should buy a helmet for only one day. It´s for your own protection. Even if you are an awesome bike rider, you never know what will happen in Thai traffic.