iOS App Signer can (re)sign apps and bundle them into ipa files that are ready to be installed on an iOS device.
To sign apps online, you're going to need the Platinum option from UDID Registrations.
Alternatively, you can purchase the Silver or Gold option from UDID Registrations to get a signing certificate and provisioning profile that you can use with iOS App Signer on your Mac.
DanTheMan827 / ios-app-signer: Our online service is powered by this open-source app for macOS.
maciekish / iReSign: The basic process was gleaned from the source code of this project.