Planing to visit guangzhou, will take hung hum train to Guangzhou.

We are taking a trip from US visit few places fist (tw, japan than arrive to HK. We plan to take the Hung Hum train to Guangzhou right after we arrived HK. How can we get the 72 hours visa? can we get it in HK airport? or we can get it in the Guangzhou train station? Please advise. Thanks You