Usually I will start by going to Amazon and taking notice of any new items that might be best sellers and work backwards from there. I firmly believe older models, depending on the niche, have their place as well as most end up on eBay. That of course is right up my alley as well.
Depending on where I am in my other content creation, I may or may not decide to farm out the product info writing to my writers. Over the years I have found that I can use my time more wisely by paying them to research and write these. There are some niches though that I enjoy more than others and will sometimes tackle that content myself. It just depends site to site.
The best thing about adding new products is the appearance that the site is being updated in the eyes of both the surfer and the search engines. If the site has any age to it and a little authority, these pages can rank almost instantly and reward you in organic and search traffic much quicker than say a brand new site.
Not every niche is a winner as I hit duds like everyone else from time to time. Start with your best producing sites and work backwards. It will take some time, but usually well worth the time to do. While your at it, have a look around at any updates or errors you might need to take care of and treat the site like you are giving it a Spring cleaning.