There probably is but it will be built on top of Mac mini or Mac Pro farms as Apple does not allow virtualization of macOS on any hardware not carrying a Apple logo (I believe that is how it is actually stated in the terms even). So of it is available it might not be a cheaper option.
But I believe Travis CI does have macOS workers available for free, but they might be scarce in available time slots.
I've been using MacInCloud at work for the past 2 years or so, renting a single server for use as a VSTS/Azure DevOps build agent server, for iOS builde. I'm very happy with it - the server we have is fast enough, both CPU and disk, and it's been rock solid.
I've only contacted their support team about billing stuff, but they were responsive and helpful.
But I believe Travis CI does have macOS workers available for free, but they might be scarce in available time slots.