Best way to feed Otocinclus is filling a bucket with rocks/pebbles/wood, fill it with water and keeping it outside. These will develop a biofilm on them (the slimey stuff) which Otocinclus love.
There's also food from Dr. Bassleer, but I have no experience with it.
Otocinclus are generally not a very good choice for a community tank, unless it's an old, aged tank. The little food there is, they consume it in a few days and often die off before getting used to tablets or vegetables or such. A group can be tricky to keep alive, because they end up being food competitors for eachother.