How do I find the percentage of customers that made a one-time purchase?

There are many ways - you can segment most of the revenue metrics by subscriptions and see the subscribers vs. non subscribers.

Start by choosing Orders in the Orders section from the main drop down menu.

From here, you can segment by changing "Segmented by" to "Subscriptions"

You can look at repurchase rate or LTV of just non-subscribers using the segmentation. On each metric you click overall and you'll be able to dig into all the segments & find subscribers in there.

Another option is looking at subscription only metrics. There is a section called "Subscription Activations" which will look at behavior between One Time Purchasers and Subscribers.