For the most part, any craft brewery in Canada or the US will fill a standard 64oz glass growler, and if they fill those they will also fill a mini keg. Here is a great site called JUSTBEER that shows a very comprehensive list of every brewery in Canada and the US, they also have a handy phone app.
While it is true that they will fill the 64oz kegs no problem, they start to get hesitant when you bring in a 5L keg. This is because they want you to drink their beer while it is fresh, so unless you have some kind of tap system to protect the beer, they are right in being worried. Just assure them you know what your are doing and use either your Pop Tap Kit, Pro Tap Kit or your Fridge Tap Kit to serve the beer with CO2. This will help keep the beer fresh for much longer than the 3 days you usually get with just pouring out of the top.
If you really want to take freshness to the next level, then maybe your favorite local brewery would be interested in setting up a Direct Fill Station to fill the keg. With a cheap Direct Fill Kit they could fill it properly the same way that home brewers do it. Here is a video showing the basic concept.