A few suggestions:

Gościnna kamienica

a ten-minute walk from conference venue

EUR 35

Orthodox Cultural Centre

right next to conference venue

EUR 23-28

Podlasie Hostel

a ten-minute walk from conference venue

from EUR 7

Hotel Cristal***

across the street from conference venue

EUR 36-81

Hotel Esperanto***

a ten-minute walk from conference venue

EUR 40-85


You can also find accommodations using booking.com lub airbnb.com.