There is really no other way to obtain this increase in faith other than through sanga. It is possible for mahabhagavatas to infuse and/or strengthen sraddha in a moment's association simply by being in their presence. But without that extremely rare opportunity, the prescribed means for obtaining the nourishment required for bhakti to grow is to regularly hear from those who have realized knowledge of the Absolute, who, although they are not mahabhagvatas, they do have the faith required to infuse faith in the hearts of others.
Collected Letters Written to Friends and Disciples Vol 4, Pg 44-45, last paragraph