I’d suggest applying a similar approach to what you’ll use to secure your proposal photographer. Also, note that it takes a unique skill set and experience for a videographer to capture a proposal. Just because a filmmaker is great at capturing, say weddings or stylized films, does not mean she’d be qualified to document a live proposal.
First, identify a handful of talented filmmakers whose work you love; you can use Google or YouTube to search for videographers near you or who are located in the city you intend to propose. Make sure you understand their work and look for some element of consistency; avoid stylized films since they allow for do-overs and are not representative of what you should expect.
If you’re using a proposal planner, ask her for 3-5 recommendations that suit your overall proposal budget. If your planner can negotiate a better rate or additional value on your behalf, ask her to reach out for availability and pricing.
Alternatively, ask your photographer, friends, or even the catering director of the hotel where you’ll spend the first night as fiancée and fiancé for recommendations. Oftentimes, catering managers have a curated list of vetted film vendors they know, like and trust.
Next, reach out to a short-list of potential proposal videographers and openly share your story, why you like their work, and be forthcoming with any additional information to better manage expectations. You might want to already mention your approximate videography budget and specify your preferred date(s) and anticipated hours of video coverage.