I assembled my own first aid kit that is roughly the same as the Adventure Medical Fundamentals. It cost about the same as the pre-made kit, but my version is easy to refill since I know where to get every item. I even have some spare items due to buying quantities even when only using one or two.
I bought some of the stuff from Walmart and Target. Nexcare seems to be the best brand of basic finger, knuckle and strip bandages as well as larger ones since they seal around the edges. I also like their cloth tape for flexibility and stickiness. I like J&J 2x2 and 4x4 Nu-Gauze pads because they're thicker. I prefer Kling roll gauze but there are other semi-self-adhering brands.
Following are the online sites I purchased from. I have attempted to list some of the items I purchased, but I can't remember them all.
Chinook Medical Gear
best prices on benzoin, bloodstopper, 2nd skin, povidone iodine wipes
National Outdoor Leadership School
best prices on SAM splint, cavit dental repair, irrigation syringe, trauma sheers, tweezers, first aid books
School Health
best for scalpel blades (better than razor blades)
Macgill School Nurse Supplies
best for Coverlet wound closure strips and other Coverlet bandages
Wilderness Medicine
best for gloves, povidone iodine, limited soft goods like Coban
Minimus
best for individual doses of OTC medicines
I hope that helps get you started. I was disappointed that I couldn't find one or two sources to get everything from, but I was looking for specific brands and products.