Add Apps from Microsoft AppSource (For SharePoint Online)

In Microsoft AppSource, you will find many useful Apps for free. Search "SharePoint Online". Once yo find the right app, click on Get it now.

Login with your SharePoint Admin account.

Click on Continue

verify your site to install, click on  Continue

It takes you to the Site's StoreFront page, click on ADD IT

Click on Continue to Confirm that you wish to add the app.

 Check 'Add this app to Home', click on Return to Site.

Click on 'Trust It', to confirm this app as trusted throughout all SharePoint Site Collections.

 Go to Site Contents >> New >> App

Click on Manage Licenses or Apps You Can Add, you will find the app and can add it to the Site.

You can also click on SharePoint Store to get only SharePoint related apps and also filter free apps.

https://spdev.sharepoint.com/sites/dev/_layouts/15/storefront.aspx

Note: If your admin has not configured your site to allow users to buy apps, you may still be able to request an app. The person in your organization who manages the App Catalog site can approve or deny requests for purchasing apps.