Contact the manufacturer of the range. Many of them make an optional filler you can purchase.

Here are some examples.

Here is a link that might be useful: Fillers

I replaced my freestanding range with a slide in range. (At some point we will remodel the entire kitchen - cabinets and countertops - but for now we needed to replace the dying appliances.) My range came with a black metal filler piece. Unfortunately it wasn't quite large enough, so I added an extra wood filler piece painted black. Find out if the range you've selected comes with a filler piece and whether it will fit the empty space. If not, and you're not planning to replace the countertops, definitely have someone fabricate a larger piece.

Thanks to you both ...

Yes, pricklypear, I'm in the same boat you were. The provided filler doesn't fit the space, nor have a I found one anwyhere on the Internet. That's how I've come to conclude a stainless L is the best solution.

I'm figuring there must be a provider out there somewhere with various sizes available. If not, I'll have it fabricated.

Again, thanks!

Could you get a piece of your countertop material and run it behind the range, with the countertop guy doing the seams so it looks like that piece was always there?

I had this saved in my favorites http://www.microtrim.com/slide-in-range-filler.htm

Here is a link that might be useful: custom trim

Thanks again, all ...

Ideagirl, yes, absolutely ... I could replace the counter section myself. It's porcelain tile and I saved a box for just such contingencies. That stated, I'd prefer an easy/quick fix rather than cutting out old tile, replacing, etc.

Thanks twosit, I will pursue that link. Thus far in looking at manufacturers of "fill" pieces, the pieces are set sizes (which is kind of curious, given the nature of the problem).

Still haven't found a source for stainless L pieces on the Internet. The saga continues!

Best ...

Any decent sheet metal shop can bend up what you want.

I stopped [purchasing the stainles splash plates to go behind stoves from the manufacturers years ago.

The sheet metal shop I use can usually make one for no more than half the price.

You might consider having a small shelf with three turned down edges and one turned over edge (folded back 180 degrees under the length).

You could then attach to the back of the stove and a ledger strip on the wall to match the turned over edge height for support.
If mounted so the top is just below the back if the stove it will not show much. just on each side from the counter up and from the back of the stove to the wall.

I have had items like this made multiple times now.

Don't forget to tell the shop the scratch pattern ('grain') direction for brushed stainless you want .. the long way looks best.