I can only speak for my country but it's probably the same in the USA. There are online stores where you can buy medication like "medicanimal" but there are some stuffs that you can't get without a prescription (in that case, you need a prescription from the vet and you have to scan it for the online store to order the medication).
It is true that most things are more expensive at the vets (the dry food or hay for example are a real rip off). The worst in France is vaccination - in some countries you can buy your own vaccines and it's like 2 or 3$ but in France the vet lobby forbids people outside from vets to buy it (even breeders can't) and thus you will pay around 45$ for it even though vaccinating a rabbit is easy as pie and that anyone could do it. It is easy to understand too : vets make a very small profit margin with operations, the most of their income comes from vaccines which are quick and necessitate no special expensive machines. Also vaccines labs have partnerships with clinics and vet schools... As a result, there is a real problem with over-vaccination. For example, many studies proved that the vaccines for dogs hold for at least 7 years (or only a few months, which make them ineffective). In many countries a lot of vaccines are done only every 3 years or so. In France it's every year for everything and we have an increase of cancers which some professionnals believe to linked to too much vaccines. The thing is that vets have studied vaccines on the pamphlets made by pharmaceutical laboratories and most of them never did a single research on their own, so they really believe that vaccines must be done every year. Others only look at the profit they make from it. And almost all of them will tell all of their clients they will kill their dogs if they don't do the whole vaccination set every year. The first vet I saw with Jeeves (my dog) wanted to do the rabies too, even though there hasn't been a case of rabies in France for almost 10 years and that the vaccine is known to have lots of possible side-effects.
So, now, I do the vaccines once every 3 years (and no rabies ever) to be sure and say to the vet that I know what I'm doing...
I have an emergency kit which mostly contains things for GI stasis as it's a common condition that can be treated if you do it early. It contains 2 or 3 small syringes (without needle), Metacam for pain, Emeprid for gut mobility and Fibreplex which is a supplement of probiotics and fibers. I also keep a small jar of baby food for force feeding (most people have Critical Care). I was REALLY happy to have all of this when Aki had GI stasis one month ago (she had that on a Sunday, of course ^^'). It's not used in France but I see mentions of Simethicone for gas very often on rabbit forums too. For sores or very small wounds I have pure organic aloe vera as it really can't hurt and it's good to prevent infection and help to mend the skin (it also reduces itches) and it's safe to swallow.