I have no idea where are the best places to shop online in the US I'm afraid.
As said above, the helmet is best shopped for in person if possible, as are any other clothes - you don't mention clothes above, but at a minimum if you are going to be spending significant time on the bike, proper cycling shorts will make it a
lot more comfortable.
Lights are going to depend on your intended use to a large extent. If you are going to be riding almost entirely on lit roads, then you can get away with much lower powered front light than you would if you are regularly riding after dark on completely unlit roads (in the former case the light is to help you be seen, in the latter, it is need for you to see). If the former, then anything over about 100 lumens (with flashing mode) should be enough, if the latter then you need at least 800 lumens, ideally more if you are going to be riding much above about 15mph (I have a 1600 lumen front light, and am comfortable at 25-30mph if I am even moderately familiar with the road). For the rear, 25-50 lumens is usually enough, with flashing mode. Get a cheaper spare as well just to get you home if your main light fails while out. Personally I prefer USB rechargeable, but opinions vary on this, many people here prefer AAA or AA type batteries (and often use rechargeable versions of those).
Pump - ideally if funds stretch, you want two. A track pump for home use, and a mini pump to carry on the bike for use if you have a puncture while out.
Lube - depending on your climate, you may again want two, one for dry weather and one for wetter weather (used all winter here). The dry lube attracts less dirt to your drivetrain, so is better from the POV of cleaning and wear, but washes off very quickly in wet weather.
Tools. If you have none, then a basic kit is probably a good idea I have
This one - not available in the US but I'm sure your local suppliers have something similar. I have yet to find anything I can't do on my bike with this set. You also want a multitool to take with you on your rides for any necessary roadside repairs - preferably this should include a chain splitter - rarely needed, but if your chain breaks, you won't get home without it.
Talking of things to take with you, something you haven't mentioned but which you should also get is either a saddlebag or a bottle tool carrier. In which you should always have a spare inner tube, puncture repair kit, tyre levers and a multitool as mentioned above. If you don't have this basic stuff with you, then there WILL be times when you are walking home or phoning somebody to come and get you.
Lock - Depends on the neighborhoods you ride, and how often/how long you are likely to leave your bike out of your sight.
Bottle holders - the cheapest plastic ones are generally Ok. Whether you want one or two depends on the type of cycling you will be doing (how long and what whether conditions), though if you go down the bottle shaped toolkit route you will definitely want at least two.