If you can estimate the date, then you might consider a search through Yahoo Finance or another financial search engine. For equities, Yahoo Finance can go back more than twenty years. If the fund's name has changed, you might consider doing a search using Google to determine prior names of the fund. If you decide to use zero as the cost basis, you probably will not be penalized for underreporting the tax as all of your proceeds will be subject to capital gains tax, probably long-term based on what you stated in your post.
@heatherm6
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