I'll warn you before hand, there is nothing that I am going to say about BC in particular, so if this doesn't interest you, you can save yourself some time by not reading on.
In Saskatchewan as I understand it, it is not "technically" illegal to collect any tree from any RM (Rural Municipality) ditch. HOWEVER, you can save yourself a GREAT deal of grief if you go to the RM and ask if you can remove trees. I am pretty sure that they would be near bending down and kissing your butt, not only because you asked first, but that you are willing to save them time and money from having to deal with them as they age and reproduce.
Asking for permission from the Highways Department, is as easy as it sounds. They are more than willing to provide a permit to you, but you have to have an idea of land location or a rough guess as to where the trees are. However (again), when you get the permit, it is a lengthy document and you have to ensure the "public safety" when removing the tree... that means flagmen, cones, barriers, etc.... and there is always a certain timeline for the work to be done. Basically, unless the tree is REALLY desirable, it isn't worth the effort.
Now Crown Land, has it's own twists (so I'm told), but it isn't a big deal. Of course anything within a Park or designated area will be off limits.
I would never go with the "just don't get caught" practice, because you can be sure that someone will happen by, and just because you want it, someone else will too. Save yourself a lot of trouble by asking someone of authority first (depending on where it is ). Provided that you have some sort of paperwork or contact info for an agent in charge, most people will back down when they want to make a big deal out of what you are doing... and if they don't (and you have the paperwork or authorization), just start to call the RCMP and they will beat a hasty retreat. I've always found that the guy who wants to make a big deal about you getting something for free, has more than one skeleton in his closet that he doesn't want the RCMP to "happen" to stumble across, over something that doesn't really involve him in the first place. I always have the Go Pro set up and running when I do my roadside work... just the thought of being recorded makes most people act in a civil manner... and if not, you have something to show the police.
The other golden lining in all of this is that often enough, the people that you are talking to have a mild interest in bonsai. I think most people have a small fascination with the art; gardeners anyway. These conversations can sometimes, turn into better material than what you were after to begin with... or a lead to something more that you otherwise would not have known about.
Of course, like all things in life, your going to trip across the typical dick that always has that stupid comment "you've got too much time on your hands"... to which I always say, "and WAY too much money to give a shit about YOUR opinion"... that usually ends the exchange.
Like most pastime activities that we start, once someone knows that you are interested in it, there always comes a time when people readily offer material to you. If you don't learnt to say "no", you're going to end up with a lot of stuff that isn't very good, or too much material to work on. Be picky... trust me, you can afford to, there aren't that many people into bonsai in Canada to have to take crummy material... and always remember that crummy nebari will always be crummy... a top can almost always be re-developed.
ALWAYS be aware of the possibility of underground telephone lines, gas lines, fiber optics, etc. A tree would become surprisingly expensive if you zipped through a fiber optics line with your cordless reciprocation saw!... and being one of a hand full of people who would possibly be digging up trees, gives the police a rather short list of suspects to visit with. (About 5 in all of Saskatchewan)
Good hunting.
