I had an internship at an American design consultancy as a Canadian, so it’s definitely possible. The toughest part is making sure the employers are informed about the visa process. Usually it makes them want to back off, but the ones who have done it always tell me that they don’t think it’s too difficult, expensive or time consuming.
I did my internship on J-1, not a TN. Most Canada-to-US interns I know did theirs on a J-1 too, but it’s definitely costly on your end (you’ll want to hire a company to do the paperwork for you, which runs around $1000). A TN would really simplify things, but the J-1 seems to be the standard for internships.