Why are all sour cream cultures I've found for sale online labeled as direct set? Why can't I reuse like I do yogurt? I've realized we go through a lot of sour cream in my household, and I want to start making probably about a pint or two a week during the summer months.

I've been researching for a few hours, and while I haven't found any reusable sour cream culture, I haven't found any reason I can't reuse sour cream culture. On the same websites selling culture, there's reusable yogurt and cheese culture, so I'm pretty sure it's not just a sales tactic. 

I've come across a few articles that state you have to be careful with reusing culture for other fermented milk recipes. It looks like contamination with other bacteria is a concern even with yogurt. Learning that, I may just go ahead and start setting aside my culture for the next batch right off the bat, before any spoon or air contamination can hit the yogurt. I learned from my parents, who reuse from the bottom of the pot when we're getting low. 

The only reason I can think of for sour cream being an issue is temperature. I assume that at 75 degrees, a lot of different bacterias can thrive. With yogurt at 110 degrees, maybe it's more of a hostile environment to unwanted bacteria, making it more easily reusable? 

If that's the case, I think I can make a jar of sour cream and pour it into an ice cube tray and freeze it to provide myself with culture for a few more batches out of that one.

If there's something special about sour cream culture that it's not reusable at all, why is it special, and can I make my own through some process?
If you can't find a reusable sour cream starter, you can use buttermilk starter.  Some bloggers and biology/chemistry professors just use fresh active buttermilk as a starter rather than ordering some online.  If you look at the various labels and product pages, you will find that both the buttermilk and sour cream starters contain the same four cultures: Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis biovar diacetylactis and Leuconostoc mesenteroides subsp. cremoris.
In the Q &amp; A for their buttermilk starter, culturesforhealth.com also provides this tip for using it for sour cream:


It is best to maintain it in regular whole milk, rather than reculturing cream long term.


On another page, Dr. Frankhauser goes on to say:


SOUR CREAM is produced by the same bacteria as buttermilk, but the starting milk product is pasteurized light cream. Bacteria are less numerous than in buttermilk.


This may be related to the lower concentrations of lactose found in unfermented milk products that contain higher percentages of fat. My best guess is that companies don't market the sour cream starters as heirloom or reusable because starting the new batch from the last batch may not reliably provide results that are as robust over time as their other products. If you just use milk for your starter, you can reuse it over time.