I got my bubble tea straws at the cake decorating store by my house, but if you have a bubble tea store close to your house you could ask them too! Bubble tea straws are a much wider and stronger than a regular straw. (they are also easier to cut than dowels...)
I have used the straws for up to a 4 tier cake, and have done the centre dowel, but only usually on a taller cake, or one that looked like it was a bit unsturdy.
One thing that was recommended to me was that if you are covering the cake in fondant, put in your 'dowels' (or straws) and then put some melted white chocolate in the space that you are going to put the next layer in. Then put the next layer on and press down a bit. This really helps the board from the cake on top to not move. I've found it pretty helpful.