One of the biggest problems I have as a stitch guide writer is having threads for a guide I just finished go out of production. My favorite Kreinik metallic is no longer made. The perfect thread for my current project is also not available any more. What's a stitch guide writer to do?
Are you familiar with the (now closed) Regal Rabbit? The owner has closed the shop and is semi-retired but her website is still open for business. Head over there now and look at the bottom of the threads page. There's a tool for finding threads built in!
The database's only weakness is that Kreinik metallics don't seem to be included. For that, visit Kreinik's online color card.
Just type in a thread color and you'll see lots and lots of alternatives to that red cotton perle you are overusing. Type in the closest DMC color number to that discontinued thread and there will be plenty of substitutes to choose from. It's a stitch guide writer's best friend! (Besides their copy editor, of course.)