It'll show up the Used Car Dealership once in a while. The only other way to get it is to use the On-line Car Dealership but since you're not hooked up to the web you will have to rely on the UCD bringing it up. You could be waiting a long time.
Seasonal events would be but as you said you're not online. Perhaps the Nurburgring 4 hour using a B-Spec driver would be the easiest way to gain credits.
To rotate the dealership you can enter and event, get on the start line and then quit. If you do this a few times it should refresh the whole Used Car Dealership.
It'll show up the Used Car Dealership once in a while. The only other way to get it is to use the On-line Car Dealership but since you're not hooked up to the web you will have to rely on the UCD bringing it up. You could be waiting a life time.
You could always do all the b-spec endurances and then grind it out of the 24 tickets awarded. I would say however, it would be easier to trade someone for the ticket but if you can't you can't! I spent a year and a bit of GT5 offline!