We migrated to Lightning and there is a custom button on the Opportunity page that loads a Visualforce page and I need to get it replicated to work in Lightning. I am not a developer nor do I have one available to me so I am looking for guidance on how to get this or similar functionality working in Lightning. Any suggestions would be appreciated.
You'll have hard time without a developer.
If the page still works, nothing's broken, it just looks classic - consider a minimal edit to add <apex:page lightningstylesheets="true"
to the top element. It'll be a bit of "lipstick on a pig" but might be just enough.
If you really need to rebuild it as Lightning Component (a.k.a. Aura) or Lightning Web Component (LWC) you'll need a proper developer. Depends on page's complexity. Some consulting company might help you, AppExchange has "rent a coder" element, try on LinkedIn maybe. You may have to show the page to a guy and get a quote before they dive in. Your situation isn't unusual, I'm helping 2 companies few h/month, they don't need a full time dev.
Complexity will depend on whether the page is old school Visualforce with viewstate or mobile-friendlier one with "Visualforce Remoting". Is it more or less pure Apex and VF or are there 3rd party JavaScript libraries in use.
If you really want to do it yourself - you're able to write Aura straight in browser's developer console but it's bit passe. LWC is bleeding edge performance-wise but you'd need proper developer tools (SFDX, maybe VSCode) to work with it.