Welcome to Geekhack!
Most Model F and Model M key caps are interchangeable essentially (IBM, Lexmark, Unicomp). There are a few artisan caps designed for buckling spring stems (some Click Clacks for example); but generally there isn't much in the way of mass produced 3rd party caps for BS keys.
Both your M13 and space saver use 'normal' BS keys/caps. Most BS keys are two piece, meaning they have a stem and cap on top, though some are one piece, like most of the keys that are more than 1.5x in width.
Example:
As far as colored modifiers, the best source for those is
Unicomp (they still manufacture new Model Ms today). They have printed and unprinted RGB Shift/Ctrl/Alt modifiers available in their webstore. If you open a support ticket with them for custom colors / widths / legends, they will normally accommodate.\
Again, welcome to Geekhack. I am a BS fan myself - as are many others here.