Search Q&A
Lats (Latvian currency)
Just go to the first banking machine at Riga airport when you arrive.
Or travel only after Jan. 1st when they have joined the Euro. ;-)
How about phoning the Latvian embassy? You never know someone there might simply be happy to exchange for you from Euro or they'd know where to exchange currency.
http://www.am.gov.lv/en/brussels/latvians-in-brussels/
Any branch of any major bank can help you as long as you warn them in advance (couple of days).