The client can use the client-side "Reimburse Expenses" tool to pay you for expenses.
Keep in mind that the same Upwork fees apply.
So if you purchase a stock video for $100, and the client reimburses you $100, you will receive $80.
You charge enough extra in your rates so that you absorb the cost of these items. And you pay for them yourselves.
You will pay $100 out of your own pocket, and you will not ask the client to reimburse you, because you are charging $3000 for the project as a whole.
You can purchase the stock video for $100 and ask the client to pay you using Upwork's reimburse expenses tool, but have the client pay you EXTRA to cover Upwork's fee.
You can send the link to the client where the client can pay for the stock video himself. Then the client sends you the file or the resource key so that you can use it in the project. (This is what I suggest that you do.)
Is it okay for you to purchase the video with your own money, and then send a receipt to the client, and have the client reimburse you directly, such as through PayPal, or by check?
ANSWER: No. This is a violation of Upwork ToS. You could be suspended for doing that.