You can buy the Snow Leopard 10.6.3 retail install DVD from Apple, cost is about $20.
http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard
And then update that, once installed, to Snow Leopard 10.6.8 by use of Software Update
from within the running system OS X 10.6.3; the Combo 10.6.8 update is a free download:
Mac OS X 10.6.8 Update Combo v1.1
Once the computer is running the full OS X 10.6.8 Snow Leopard, it may be able to go
online and you could buy an upgrade to OS X 10.7 Lion from Apple online. Or stay at
Snow Leopard 10.6.8 and it should be OK to run the older Apple applications.
If you later choose to upgrade to Mac OS X 10.7, that also requires an update from Apple
servers, from the App Store. The actual upgrade file would be a download after you buy it
and requires Snow Leopard 10.6.8 to be installed, & up to date, before Lion can be...
•OS X Lion (10.7) - Apple Store (US)
http://store.apple.com/us/product/D6106Z/A/os-x-lion
Given Snow Leopard 10.6.8, your older Intel-based Mac could still run some older PPC
coded applications that otherwise would see no support once past this OS, to 10.7.x
Snow Leopard installer includes a now-discontinued 'Rosetta' utility to help run old apps.
With correct and positive identification of the hardware, the software
items, utilities, and accessories are easier to identify, locate & buy.
Good luck & happy computing! 🙂