The red diamond shaped Altezza emblem is a part of the grill, they are not sold sepearately. Moreover, I don't think you can take the emblem off the grill in the first place. The dealerships only sell the grill itself. The stock grill will cost around US$300 I guess. You'd have to buy them as service parts, hence the high price.
The rear emblem will be around US$7-10. But this is just a guess too. If you take off the IS300 letterings, you can certainly stick the Altezza logo. Both IS300 and Altezza emblems are just stuck on the panel with a strong adhesive.
In order to get these parts, I guess some customizing or tuning shops in the US will be importing these Altezza goods in the near future. (Maybe some already have.) Or you can ask someone living in Japan to buy it at a local dealership and send it to you by mail.
If you're wondering if I can help you, I'm posting a message on the Altezza forum.