There are several places that sell additives to make it taste more sour. I know having an older thinner starter can get more sour in time ,but what else?
lactobacilli can it be added to a starter when and what type?
My starter is flour and water. The fermentation process creates the lactobacilli. Each starter is unique to its own environment, i.e. the bacteria in the air and utensils and are part of the fermentation process. For myself, I would never add anything. The whole idea is a natural process. Additionally, sourdough/natural fermentation takes time. It is a slow process compared to commercial yeast baking. That is part of its charm and ultimately the flavors.
I’m not sure there is one answer for everyone’s starter to make it more sour as I gather that is what you are looking to do, but you can search on “feeding for more sour”. Feeding more or less … can’t remember … can add sour flavor.
My own starter tends to be not that sour. I love my breads and baked goods so I don’t fuss. Per your other question about taste and smell: I’ve never tasted my starter. After being fed and bubbling up it has a pleasant yeasty smell. But I imagine that everyone’s starter smells slightly different and we all perceive the smell differently.
Sourdough/natural fermentation is NOT an exact science 
There are so many variables: type of flour, age of flour, water, and then the mixing and baking and ovens and weather. There is a huge thread on maintaining starter here on Breadtopia as well as other questions asked and answered - good resource. I started several years ago with some posts on thekitchn.com and use that method of starting and maintaining my starter (search sourdough on thekitchn and look for Emma Christensen’s posts).
FWIW, I use my own unfiltered, untreated well water and a local to me (WheatMontana) minimally processed flour for my starter. I live in the woods in NW Montana and have a dog and cat. I imagine I have a great amount of interesting bacteria contributing to my starter and anything else I ferment ('kraut & kefir).
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Haven’t made kraut in forever! Thanks I think I am hurrying things. Time will tell. I have the rise, so thats a start.
Snow in August. You can keep it awhile 
My husband and I were just having a similar conversation about my sourdough just the other day. My sourdough, Cyril, is not what I would call “sour.” But that works GREAT for us because I simply want to bake good tasting bread without commercial yeast. Cyril gives me lovely loaves of tasty bread.
I think I will do the overnight rise in the fridge and bring it out for the final rise. They say that helps with flavor.
@Crystalview, I have read from a number of bakers that technique seems to work for them to achieve a more sour taste to their bread. Go for it! Please let us know how it works for you.
Using a whole wheat levain will make it more sour, also using the fridge for a cold bulk ferment and/or cold proof will make it more sour.
You can add a tsp of citric salt to the dough. It is a granulated Salt. It is used in cheesemaking and will give your loaf a sour flavor.
My starter culture is now almost 5 years old (time for kindergarten?), and is maintained from flour and water. Which is what I use to make my bread, though I use whole grain flour (I buy the grains here, I haven’t yet found a local supply) which I grind myself (a Kokomo mill, which I also bought here). My husband and I love the flavor or my bread – but my adult daughter found it too sour! I make a levain from my recently fed started culture that I then ferment for about 4-6 hours, and add it to my dough, which I also ferment about 4-6 hours – but the times vary, and I frequently put either the levain or the dough or both into the fridge because I forget what I’m doing or that I had other events on my calendar. (LOL!!) I then proof the loaves about 1.5 - 2 hours; I haven’t yet put those in the fridge, but I’m sure I will at some point. Luckily, sourdough is very forgiving, and all the bread delicious!
Thank you. My pancakes were sour and yummy. Think I will not use DS powder sounds like is mostly for levining
I keep my starter in the fridge and it gets pretty sour, depending on how long it sits. I just add flour and water to bring it back to life, letting it sit on the counter until it starts bubbling again.