I don’t know why this posted as a poll. I’m just looking for a solution to the design dilemma…

Have a look at threshold pieces. The tile stores will sell them as well as Home Depot and Lowes. They come in different stones, colours, sizes and widths.

You didn't say what dimensions you need. If all else fails, visit a stone yard and ask them to see their remnants and they can cut and polish what you need.

You really would ideally want it to be a solid surface with no grout joints etc. This would be a better choice than tiles.

Have a look here - these are what you are looking for.... but good tile stores will sell lots of sizes, widths and options.

Thresholds at Home Depot.ca

The sill needs to be 4&1/2” wide to cover the edges of the vertical tiles. The whole length of that side of the shower is 58”and the doorway opening is 25”.
Part of the problem is that the nearest Home Depot or Lowe’s is 240 miles away. When I look online, I’m finding nothing on either website. So frustrating!

I have looked at so many tile stores online… I’m finding nothing…

This is what I see when I do a search for thresholds at Home Depot.

Are you located in Canada or the USA? The link I included was from Canada (that's where I am). Are there some tile shops in your area that you can visit in person? I know some of the tile shops I go to have pieces displayed in various sizes and widths but maybe they don't show them on their website.

If not, then is there a countertop fabricator or stone yard? You can use stone or solid surface like a quartz piece of countertop and get them to cut the size you need and polish the edges. They will likely have a remnants section and you can hopefully find a piece that will work with your tile choices.

I found This one on Lowes.com. 6" wide and 84" long.

It may not be the colour you want/need but there are others on the site. Once you link to this one, you'll see the others.

Also Anatolia Tile has marmiline thresholds and other shapes in lots of sizes and colours. Maybe you can locate a tile store that carries their product that can ship to you.

Please go to a real tile showroom; they have them. Sill come in 6x72 size, with finished edges. They are available in travertine, marbles, quartz and granite.

I just a custom shower done in a new build. The shower and floors are in a stone look porcelain. I know it is best to use slab material for threshold etc., and flooring guy recommended Corian. It comes in so many shades that we were able to pick the perfect color to go with the tile. They used the same material for the shower niche, so I'm thinking this would be a good solution for your sill as well.

Thank you, all of you, for the ideas and leads. I pretty much have to buy online- I live 80 miles from the nearest normal grocery store and 240 miles from the closest Lowe’s or Home Depot. Theres one tile shop there I have yet to call… There’s only one stone yard there which serves most of southwest Texas where I’m located, and I’ve been on the phone with them today to see if they might have a remnant that would work. You’ve given me some things to follow up on; thanks.

Tile on the flat sucks. Have a color matched solid surface sill fabricated and installed please.