i only have the original game rct2, no expansions. and there are only 10-15(?) missions... waaay less than the first 3 campaigns. so do people make their own and play those? or are most of the good missions in the expansion packs?

Building your own is only one option. You can also download some of the hundreds of scenarios which are to be found online. My personal recommendation is to get the 50+ scenarios from RCT1 and it's expansion packs rebuilt for RCT2 at rctspace.com (i added the link to my post somehow). Have fun playing ;)