Hi Patti,
Start at the top. Go to amazon.com and use their search engine. You can just enter Joan Bennett or you can enter Joan Bennett and use their drop down menu to look for specific media types, such as DVD or VHS, etc. That's where I always start when I want to see what's available for purchase.
There are 45 selections under Joan Bennett DVDs but not all of them are Joan movies and some (ie: Father's Little Dividend/Father of the Bride) are listed multiple times.
There are 382 listings under VHS; Again, not all Bennett movies. And it appears that most, if not all of the DS series is listed.
Needless to say, I'd have a list of her movies from the IMDB in one hand to compare with Amazon's listing to get a quicker picture of just what's out there.
There are obviously more on-line stores to search, I just always start with Amazon because they're the biggest and I am guessing, the most comprehensive.
PS -- Have a fab weekend!