You are legally allowed to take firewood back and forth across the Canada/US border and have been doing it for years. The stipulation is that it must be dimensional sawn lumber without any bark whatsoever on it. Split rounds of firewood is absolutely not permissable. We keep a small amount in our truck for the many camping trips we do in the US each year. We get pulled over at the border by the US border folks at least once a year for a "routine computer-generated" inspection. Once, they went through every stick of firewood and found one piece of 2x4 with a small amount of bark on it and they confiscated it. Pieces of cedar for kindling are also okay as long as it is clearly sawn wood that has been processed in a mill. If in doubt, check on CBSA or CBP websites.
The Canada and US border agencies don't care if Canadian and American wood is mixed together, at least from BC to WA. Other parts of the US and Canada have different species of wood and their can be restrictions on it's transport.
We bought a large truck load of seasoned firewood at Perry Pallet co., east of Birch Bay. They have a mountain of pallets they saw up into firewood sized pieces about 16" long. We sorted through it and were able to get consistent similar lengths that allowed us to pack the wood in tightly. It was mostly 2x4 & 2x6 while some was 4x4. It cost us $50 for about 3/4 of a cord. Wow that was a heavy load!
The thing about firewood is that unless you are a hardy outdoors type, it can be too cold & wet for a fire and over the summer period there's often a total fire ban, so a large load of wood can last a long time. Some locations don't allow open fires but the portable firepits with a screen cover are okay. We've noticed in recent years that some CGs have removed their firepits altogether, which in some cases could just be to reduce maintenance. We also have a propane campfire in a can that we occasionally use.