There is a Seattle company https://www.themedragon.com/ has done a reasonable job for us recently. It costed me about $1600 for an extra length video and voice over. They can do turnkey job.
I have used fiverr.com before. It is cheaper but you have to do the project management yourself.
1. Draft the script
2. Script review https://www.fiverr.com/ccutting
3. Voiceover https://www.fiverr.com/linneas88 (female), https://www.fiverr.com/markc1542
4. Whiteboard animation https://www.fiverr.com/steveeyes, https://www.fiverr.com/uniquefivex