I do not go first to Amazon as they are often not the best price and service but I do a search for anything specific that I might want, find the options and look if any are closer, cheaper or fit the situation better. Be aware that there are tons of small operators who may do far better than the more known folks. From there it is not hard to have things shipped to where you might be but it does comes with risks. Right now we are looking at lots of delays in mailing and the dealers don't always have things on the shelf, so if you are going to only be in a campground for a week and it takes a week for the dealer to get it before mailing to you, then you can create a bit of mess!
Many items are things I want to pick up and hold before buying so we make it part of the slow trek to go by and look at small dealers if they are close to the route. Just a Google search and locate is good for us but it does pay to sort them closer as some may only be selling out of the back bedroom and not have brick and mortar places to look!
We often do a phone call to check!
Do they exist as you head West? Depends on whether you start from New York or Seattle!