FLOUR
Bread Flour - very important to achieve the big oven spring. Bread flour has a higher protein (gluten) percent. Higher the better… when baking sourdough, the protein/gluten is what the sourdough feeds on, giving it a bigger rise/ oven spring, (The height of the bread after baking). If you are gluten free dont let this scare you… sourdough bread is super low on the glycemic index. HERE'S A BLOG EXPLAINING THIS.    
However, If you have a gluten allergy proceed with caution (in my findings with a few of my friends who have gluten allergies they are able to eat sourdough safely with out having issues… (they say they can only have home made sourdough, store bought, they canNOT, probably because most store bought sourdough is not “real” sourdough.) with that being said I am not a doctor or health professional… so just be careful and listen to your body if gluten is an allergy for you. Also if you are in need of real gluten free sourdough there are blogs and recipes on how to convert your sourdough to gluten free using GF flours. I have not ventured into these waters yet, simply because I don't have a need for real GF.

King Arthur High Gluten Flour (aka King Arthur Sir Lancelot) - this is what I use and highly recommend. not easy to find, the highest-gluten flour around (14%), it's key to helping you bake lusty, chewy artisan breads and whole-grain breads with a hearty, lofty texture. It's good for breads that require long fermentation (sourdough is long fermented). LINK

King Arthur High Gluten Flour (aka King Arthur Sir Lancelot) - this is what I use and highly recommend. not easy to find, the highest-gluten flour around (14%), it's key to helping you bake lusty, chewy artisan breads and whole-grain breads with a hearty, lofty texture. It's good for breads that require long fermentation (sourdough is long fermented). I buy 50lb bags of the Sir Lancelot from our local restaurant supply store. (If you are friends with a restaurant owner ask them if they will add a bag to their next order… this is the least expensive way to purchase this flour) there are some restaurant supply websites that have it but you will be paying extra for the shipping.

For bulk 50lb bags online, BAKERS AUTHORITY, AMAZON

Other flours… there is so much science to flours and baking I have spent weeks/ years researching and trying to understand… all have their purpose in bread baking… have fun and play around… here are some suggestions of other flours to consider.

King Arthur Bread flour - easy to find at most grocery stores, 12.7% protein, a whole point higher than other bread flours on the market. I ONLY USE THIS ONE IF I'M WAITING FOR MY HI GLUTEN FLOUR TO ARRIVE.  LINK

Rice flour - a must for dusting the proofing basket. Should be available at your local grocery store (Krogers, Whole Foods, etc... should have)… this link below it way over priced on amazon if you buy just one, the box of 4 isn’t too bad (works out to be 4.59 ea) but thats the brand I like. Any rice flour will do. If you have a high-power blender (vitamix or similar) you could even make your own using white rice. LINK

Whole Wheat Flour - easy to find at most grocery stores, 13.8% protein. LINK

Rye Flour - easy to find at most grocery stores. LINK

Caputo 00 Pizza Flour (red bag) - most specialty grocery stores carry (whole foods, or an Italian market, etc.) this is a must when making pizza, the red bag is for home ovens (This flour is great for home ovens up to 500 degrees Fahrenheit), *blue bag for High altitude commercial ovens (Milled specifically for use at temperatures above 700°F) buy the RED bag. LINK

King Arthur Sir Galahad (All Purpose flour) - perfect for any recipe that calls for All Purpose flour. 11.7% protein. This is my favorite! I buy this is bulk as well. LINK TO 50LB BAG, LINK TO 5LB BAG