Hi Joe,
The free App Service managed certificate isn't exportable. Some options to consider are a) obtain a certificate for free using Let's Encrypt b) purchase certificate from third-party provider (costs less than App Service Certificate) c) purchase App Service Certificate as you already mentioned.
Let's Encrypt (LE) is cheapest option, however, there are some downsides. First is, if you have never set up your machine to request and obtain certificate from LE, you will need to spend some time reading/learning. Second downside is the certificate will only be valid for 90 days. This isn't a big deal if you set up scripting to automate the process of obtaining/installing on schedule, but if you are doing everything manually the work will need to be done every 2+ months.
Compare this to a purchased certificate which is valid for about 13 months typically.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP