re: Using straws?

I won’t be making a cake more than every few years, if that, so I don’t want to invest in any sort of system. But…I also don’t want this cake to fall either! The thick tea/bubble/milkshake straws are what I’m going to have, and as suggested I’ll use the cardboard (didn’t think I’d need it but if I do, I do!). round hollow tubes are VERY structurally strong, they ought to be more than capable of supporting a cake unless I get really thin flimsy crappy ones. So…just 2 more questions…where can I get an 8" cardboard for the cake, and do I need to bake the cake with it, or set the cake on it afterwards?
I’ve never done a cake that was more than just two layers thick before (well, I did it once…but the other lady stacked it and iced it while I cleaned up, so I missed how it was done!), and want this to come out great! The wedding cake for my wedding, I just made the gum-paste decorations and didn’t do a thing with the cake but eat it.
Thanks for taking the time to help me on this!