You can generally use a plain steel F string to complete any 7 string set.
I'm not aware of any bass ball end or length gold colored plain strings.. actually.. Aurora has gold plain steel... They are closed till after New year's but I should be able to make that single happen.
Yes I know high-F plain string can be interchangeable across brands but we're talking gold color scheme here, and I've just checked your website, there's no Aurora singles as an option and their sets don't even have as high as high-C string.