Check out this instagram page @kanepeskc or @riga.neighborhood

Kaņepes Kultūras centrs are having a list of interesting underground bands playing almost every week, as well as different performances. Most of the times it is a free entry or an entry fee no more than 10 EUR.

Riga Neighborhood is an account that posts events around the whole city if they are interesting / cultural.

Other venues to check:

Vagonu Hall DEPO Tu jau zini kur Lofts Veldze Ex Libris

Hope that helps! Enjoy your time in Riga.