A rather cheap solution is to buy little soccer balls made from foam and cut them in half. You can even adjust the shape if you're handy with a knife for a nice teardrop look.
Not the most authentic look and feel, but if you want something to anchor a nice chest harness without risking it getting damaged by tight ropework, it's serviceable. Put some double sided tape on the cut surface and stick a plastic sheet (a cut up ziploc bag) to it to avoid getting the foam soaked in sweat. Ideally you will use a bra or bandeau to hold them in place and cover them up, since, well, they're foam soccer balls and don't look like breasts at all. But the shape and squishyness are quite nice for adding boobs to your selfbondage experience.
I got 3 for $20, they are available in different sizes, with diameters from 4.5" to 8".
Edit: Just realized I already made
a post about it in 2021
Life's too short to pretend you're not into some kinky shit.