You can try to modify one of these supporting panties by cutting the legs off and making a hole in the middle
to fit your head through only just, it mustn't be too wide or otherwise the flattening effect won't be good enough.
I know it sounds a bit weird, but they're a very cheap alternative and very comfy too.
I bought a couple of super cheap ones, sacrificed a few by cutting too far and too wide, but finally got it right and
make some new ones as soon as the old ones wear out or don't fee nice anymore. You can also buy more expensive
ones (30 EUR), but be surer you've practised on the cheap ones before;))
I read about this somewhere on a transmen page, but forgot where, so just tried it out for myself and it works a treat!