See this little bag holder:
They cost about $7 to $10 and just attach to the handle of a leash. We have a couple of them and keep a stock of baggies in each vehicle as well as in the bag holder itself.
I've often offered a baggie to someone who isn't picking up after their pooch as a "niceity" - "Here, would you like one of these? I'm happy to share if you don't have any..." Just because I want to be able to keep taking my dog to the parks and other areas we like to roam. We're careful to pick up after him but not everyone is - and we don't want to lose the privilege of going to our favorite spots because some people are inconsiderate...