Home brew supply stores will/should have them. I got one from the LHBS not long ago and also see that AHS has one listed. I'm sure a HBS in your area, or one you already order from, has one. Just confirmed that Rebel Brewer also has them listed.
I prefer to get things like flasks local, unless they're guaranteed from breaking (or the supplier will replace if it arrives broken) during shipping. That way I don't need to worry about getting a box of glass shards instead of a flask.
IF you're not going to heat the starter wort in the flask, then you can use a variety of items as one. Do a test with the stir plate to see if the item functions properly or not. If you already have said items on hand, then why not try them?
Personally, I have a 2L, 3L and now 5L flask to use for starters. I also have some 1/2 gallon mason jars, and one that's just over a gallon in size.
The one thing I will suggest you do is get at least one strong rare earth magnet. Use that to hold the stir bar, or remove it from the flask, before you pitch the slurry. I use that to remove the stir bar before I decant so that I can then get the yeast into suspension in what's left and pitch it.