Here are some links from referred places from Mizuno's Website for single irons. Just looking at the top one, it looks like Ironfinder has a 3i in a regular flex TT steel shaft in "Good" condition for $49 (see the very bottom link on the post to go straight to the club). If it is the wrong shaft, you could check the others or reshaft it? Good luck!!! May want to check the others if you want one in better shape.
MIZUNO USA .: FAQs Do you have replacement clubs for your older models?
Once a model is retired we keep a limited inventory of replacements for approximately 1 year. Here is a list of companies that also may be able to assist you.
Iron Finder /
Ironfinder Golf: Quality New and Used Golf Irons / 877-346-3372
The Golf Warehouse /
TGW.com - The Golf Warehouse - The Premier Online Golf Superstore / 888-838-5551
Edwin Watts /
Edwin Watts Golf - We carry golf clubs, golf balls, golf accessories, golf shoes and golf apparel from Callaway Golf, TaylorMade, Taylor Made, Titleist, Cleveland Golf, Cobra, and Nike Golf / 800-874-0146
2nd Swing /
The New 2nd Swing
Club Finders /
::: Clubfinders Golf ::: Used Golf Clubs , Individual replacement single lost golf clubs. / 888-234-6325
Club Scouter /
Used Golf Clubs & Golf Irons : Lost Golf Clubs & Golf Irons : Jon's Golf Shop in San Antonio Texas / 800-749-6207
Old Sport Golf / 770-493-4344
Ironfinder: Available irons