I've been to the Aegean Plaza at Kamari and it was an excellent hotel and was also flat. So this should do you alright!

Thanks qman I will look in to it
:-)

I'd second Kamari as probably being the flatest resort. We really enjoyed our time there.

Thanks. Looks like Kamari it is then ✈️

There are two beach resorts on Santorini. Kamari and Perissa.
Both resorts are flat. Kamari tends to have more beach front hotels than Perissa, which tends to have more SC options.
The resorts on the Caldera, (Oia, Fira, Immorvigli, Firostavfani) are not beach resorts and a very 'hilly' but this is where you will find all the picture postcard views. Accommodation in the two beach resorts is much cheaper, and probably your best bet.
Hotels and SC apartments can be booked direct with the owners, or through websites like booking.com, alfa romms etc..but unless you a very lucky with your flight price, it would be cheaper to book via a travel agent either in store or online.