Seb Cot
Upwork.com or fiverr.com are the sites I usually go when I need help from a virtual assistant.
Before you hire a VA, go ahead and do a few product descriptions yourself, so this way you will have an idea of how long the process takes. Once you have you know the average time it takes, place an ad on one of these sites and try 2-3 different VA's.
Give them a few products to list. Ask them how long it took them. If the answer is close to what you experienced when doing it yourself, hire the VA do do the remaining products.
Basically you have two options... Pay them an hourly rate or per product (my preferred method).