Right now at Costco (at least mine here in CA) there is a first aid kit for $25 that (in my opinion) is a good deal for a basic "boo boo kit". Its got all the basics and it comes in a folding hard plastic case. If you wanted to, you could pull out the most useful ingredients and put them into a soft MOLLE type pouch that could be stored in the vehicle more easily. I tend to roll my own, but I also carry a first responders trauma kit (with sutures, intubation kit, clotting bandages, splints, pain meds, staple gun, etc).
I'll second the REI packs. I probably own 4 or 5 of them and they stay with the gear that is used for that activity - for example, a small one stays in my mountain biking bag and a larger one is in my hunting backpack.
Note that if you roll your own (or buy pre-made sets) don't go cheap on the bandages - especially the adhesive bandages (which lets face it, get used 90% of the time vs, 10% for everything else). There is nothing worse than crappy band-aids that won't stick or leave a gummy residue!