There is some information available. If you're looking into the literature, probably the best place to start for scholarly publications is the EconLit database. You could also consider keyword searching in the Factiva database for news related to microfinancing. Please consider multiple keywords that may be relevant (such as 'microloans' as an alternative, for instance). One database that might have some relevant statistics is the Statista database. Lastly, the IMF data portal includes something called the Financial Access Survey (look in 'datasets') which includes some information about microfinance availability in the covered nations.
There are also some free resources that may be useful. USAID maintains the Microenterprise Results Reporting tool which provides limited information on specific projects. The International Labour Organization maintains a LibGuide with relevant links to resources. And there are relevant publications on the FinDev Gateway and Global Development Research Center sites.
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