Little is available on line for booking...which is understandable if you are renting rooms for under $15 US Dollars per night...
The main street has many cheaper losmans and small B&Bs...
The main street, Jl Raya and especially Monkey Forest Road are the traditional places to look for a low cost place to stay, but recently many new low and medium cost places have sprung up in Jl Hanoman and Jl Bima.