At Wayfair you can search by measurement: Here you go.

You might be able to cut down a rubber backed bath rug without worrying about raveling. The shag type would hide the cut edge? Just a thought.

Partim, Are you suggesting that I get a carpet tile and cut it down to 18 x 24? I’m not sure what you mean.

Yes

I looked at Joss & Main, Overstock and others when looking for my powder room. I also check Home Goods for rugs but didn't find any good small ones. I saw some smaller than than 24 x 36, but not sure I saw any as small as you need. I think you need to look at bathroom linens for a bathroom rug. I just checked and Macy's has 17x24 in Charter Club and 18x25 in the Hotel Collection for starters.

If you don't need to be able to clean it, you can get a carpet store to cut and bind something for you -- possibly from a remnant or a sample.

17x24 is a standard size: 17" x 24" bathroom rugs.

IKEA. 16x24

I have great luck finding very small rugs at Ross, Dress for Less...sometimes for less than $10!

Are you looking for a bath mat or more of a rug?

The Bed Bath & Beyond website shows bath rugs that are 18" x 24."

https://www.bedbathandbeyond.com/store/s/bath-rugs/_18-x-24/c19mX2JpblNpemU6IjE4IiBYIDI0IiI=?pstate=pv_view:grid||&ta=typeahead&ml=v2

Lots of choices here:

https://www.landsend.com/products/essential-cotton-reversible-rug/id_212982?sku_0=::NAV

and:

https://www.bedbathandbeyond.com/store/category/bath/bath-towels-rugs/bath-rugs/13435/13435-4294966331-800057100/1-48?pagSortOpt=DEFAULT-0&refType=true&_requestid=5558399

Since it is just a powder room next to a bar, I am looking for more of a rug than a bath mat.

Search for a carpet company in your area who does binding and find a suitable carpet remnant then have them cut it to your exact size and bind it.

Try www.esalerugs.com Here is an example https://esalerugs.com/light-brown-1x2-modern-ziegler-area-rug-22314132 it's 16" x 24". They have lots of unusual sizes. This company is also a pleasure to do business with.

18x24 rug @ kohlsKohl’s. But also google 18x24 rugs

18x24 rug @ Kohl’s

I never use a rug in a powder room

Maybe consider an entry mat, if you want something more rug-like? They come in a variety of colors and patterns, not just the rough "mudroom" mats. Because I lived, for years, in a condo with carpet in the common hallway and a "rough" entry mat outside my door, I had one that was pretty and soft inside, just to set wet shoes on and make the entry hall look complete. I think I bought it at Walmart? And it was pretty much the size you're looking for...if it wasn't 18x24, I would be surprised.

I have bought several rugs from Main Street Orientals in Maryland. They are very reputable and carry only high quality items. I would not hesitate in buying from them and having them ship something to you. They have several on their website that fit your measurements and have a sale going on. See them at https://www.mainstreetorientalrugs.com/collections/2x3

As regards the comment about never putting a rug in a bathroom: I wash my less expensive small orientals myself - gentle swishing with Tide in warm water and a lot of rinsing with cold water. I put mine in the tub and after washing let the shower run for about 5 minutes on them to rinse.