You can try manually loading font over https, which Google Fonts also supports.
ui = bootstrapPage(
tags$link(
rel = "stylesheet",
href="https://fonts.googleapis.com/css?family=Source+Code+Pro"
),
tags$style("h1{font-family: 'Source Code Pro'}"),
tags$h1("Test Font"),
numericInput('n', 'Number of obs', 100),
plotOutput('plot')
)