You ask : How best to reach Soomaa by public transport from Riga?
We answer: Take a morning bus from Riga to Pärnu. It takes two and half hours. There are various buses, see www.tpilet.ee for options.
From Pärnu there is an afternoon bus leaving 2:30 pm to Riisa. It takes one hour to reach Riisa. Riisa is a village inside Soomaa national Park.
We will meet you in Riisa bus stop if agreed so.