Where can I download a trial version of InDesign CS3?

On the system you have I can see one of two ways to go. I just looked on eBay and saw somebody selling an old copy of CS3 for $50 and swear that they have tested it and it works. You should know that the servers that confirm the serial numbers for CS3 have been shut down but it is possible that the copy they are offering comes with a serial number that will work locally—for $50 it might be worth the risk. The other way to go is to get a subscription now to Creative Cloud (which it sounds like you were ultimately intending anyway). Once you begin your subscription you will be able to download any versions of the Adobe programs from CS6 on. As it happens CS6 is compatible with your Mac 10.6.8 so it would solve your current problem. Once you are ready to set up your new mac you are all ready to go since a subscription to Creative Cloud allows you to install on two computers (and you can always deactivate the copy on your old computer anytime you want). I think the second way to go may be the better way to go.