I have to disagree with NatalieMarie when it comes to using a support in between tiers. You should always use a cake circle or foamcore underneath each tier, even just 2 tier cakes. Otherwise you will likely end up with bulging on the bottom tier. In addition to providing proper support for the top tier, this will make disassembling the tiers much easier and less messy.
Also, with regard to the supports within the bottom tier, you can use dowels, bubble tea straws, SPS, or even drinking straws if that's all you have. If you plan on transporting the cake, I would recommend inserting a long dowel rod down the middle of both tiers to hold them together better during transport.