There are probably hotels in New Phimai, but I did not even check since it made more sense to stay in the old town just in front of the ruins and near Saingam village. In such a way, you can enjoy the sights from earlier till later.

Since the publication of my ancient travel guide (1992! And it looks like a 1940 edition), the Old Phimai guesthouse has been unified with Phimai Beer House, and both buildings, one in front of the other, are managed from the "Old Phimai" one. The structures are really basic, providing just the basic commodities, but at a price of 130 baht per room (a little more than $3), you cannot complain. As very few tourists arrive here, you will probably have the building for yourself (I had), thus adding a feeling of extra luxury to the experience. The buildings are classical wooden Thai structures and not very well kept, giving a weird sensation that you stayed to sleep at the Phimai ruins themselves. The staff is cordial and helpful, they rent bikes for the tour to the Saingam Village, and they even sent their little girl to show me where I could get pressurized air for the bike wheels. Despite that, they provided erroneous information regarding the day pass to the nearby Khmer ruins in Cambodia.

If that option is too rustic for your taste, just walk back from the guesthouse to the main road, take to the right, and take once again to the right at the wide road and you will find in front of you a regular hotel, albeit with only basic facilities, with rooms starting at 350 baht (almost $9).