Custom Bootstrap stylesheet sources
https://bootstrap.build/ - Free Bootstrap Builder (Bootstrap 5, so for Tiki 25 or newer)
https://themestr.app/ - Bootstrap Builder, Themer & Customizer (Bootstrap 5)
https://bootsnipp.com/builder - "A design element gallery . . . . Find snippets using . . . Bootstrap."
Bootstrap Magic - Input values for variables and see effect in real time; can export Less variables or CSS (for Tiki versions using Bootstrap 4).
(There are many more of these sources. They haven't been tested specifically regarding Tiki.)