I used this one. It was a life saver in the first 2 weeks because it hurt my stomach muscles to lay completely flat. With the wedge pillow, I could sleep with my upper body a little elevated and not stretch my stomach muscles or belly button incision. Later on I was able to flip it and use it as a back rest when I wasn't ready to sit at a 90 degree angle all day for work.

https://www.amazon.com/dp/B00DTZVLTW/ref=cm_sw_r_cp_apa_glc_fabc_3Y5BDTQ50JAS03WCCT8E?_encoding=UTF8&psc=1

I'm sorry, I do not know how to find out if this is available in Japan, but here is the one I have (the 10" one) and it is well within your budget.