Unfortunately it is very hard to find Mizuho for sale now. Most of the Mizuhos become collectibles and they are not available for sale at all. In rare cases they are going very high...
As the starting point with Mizuho radios I can recommend to looking for MX-21S which is the best choice for on-air operation/DXing I guess. Also there was huge production of MX-21S I guess so this model is not as rare as MX-14S or even worse MX-18S or MX-24S, MX-2F etc.
Sometime you can find Mizuho on eBay, but nothing currently available if I remember well.
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Also please try to check these swappments:
The most important when you are looking for Mizuho radios is:
Hope it helps a bit.