I don't think there ever was a digital version of GTR2. It was expected from SIMBIN to release such a version (there was/is demand) but, for unknown reasons, it never happened.
You may find the odd CD/DVD original copy available in the bargain bin of some local shop, but it's extremely rare now.
It's through online-retailers where you'll find this cheaper and available in bigger numbers. In most cases these ship worldwide (shipping costs may or not be included depending on seller), and you'll be able to pay with Paypal, Credit-Card, etc.
Beware of overpriced copies (especially in the case of the "G.O.T.Y." edition), the games are now extremely hard to find but it's silly to incentive purchasing copies from opportunist sellers.
Here's a personal note: used copies are also a great solution.
People think buying used games is a wrong aproach, and that there will be problems when time comes to play GTR2 online (and P&G too). So long as the used copy of GTR2 works fine, this should NOT be a problem.
In the worst scenario, you can get a serial-key to play online (logged in the SIMBIN lobby) with your used copy, for FREE, with just a valid e-mail register being required on certain websites (NOTE: not sure how long these nice promotions will last).
Here's one of those websites:
Searching for GTR2 copies online, this is what came up:
There's also the "G.O.T.Y." edition, the last one released for sale (late 2007) where both GTR2 and GTL are sold in one pack (two separate games on one single box):
GTR2 - GAME OF THE YEAR EDITION:
Hope it helps!