Wall grille that fits one step stair
As with many other things (a question about thermostats comes to mind, where various higher-rated thermostats are used simply because it is cheaper to spend an extra $1 on the heavier switch than to stock 2 different items), limiting the number of SKUs (stock keeping units) is important in a lot of businesses. So you won't find a big box home improvement store stocking every possible register size, just the most popular.

Four solutions:


As @bib suggested: Use the shorter one and add a wood filler strip. The only catch is that if the shorter one is too short, you might lose some airflow, but it would probably work just fine.
As @Alaskaman suggested: Clean &amp; paint the one you have. If the problem is cosmetic, that is the easy fix. A little more work but no worries about matching up the screw holes or the overall height or anything else.
Try an HVAC distributor. They are typically wholesalers and will charge walk-in customers more than they charge their regulars, unless you can convince them that you are a tradesman (perhaps a handyman who only occasionally needs HVAC parts) and not just a homeowner. But even at 2x price, it won't cost much. If they are reputable, they won't sell you refrigerant or a whole furnace/air conditioner. But they'll sell you registers (which is what you need), tools, filters, small parts, etc. as long as you know what you are looking for. In this case, I'd bring the old one to the store (which will likely be off the beaten path with a bunch of warehouses or car repair shops, not a shopping mall) and see if they can match it. Since they deal with people building or renovating houses &amp; offices and not just fixing little things, they tend to stock a larger variety of items.
Special Order: Some big box stores have catalogs of special-order items available. It might take a week to get, but with "deliver to store" there is often no extra shipping or special order charges. Ask at the "pro desk".