in this ac is converted to dc and then converted coverted back to ac.i found a component RDN240/20 which is a varistor ..i like to know abt its availability
The RDN240/20 is a MOV - Metal Oxide Varistor, that clamps at 240v.
You don't have to use that exact part; just a MOV with a similar rating.
That design is not very good, as there is no "real" regulation. It will not be very efficient.
Since I don't know where in the world you are, suggesting a vendor is difficult. People in Europe tend to use vendors like RS Components, Farnell and Maplin.
The worst thing about that circuit is the battery connected straight to the rectifier output. If it is fully charged and the mains is a bit higher than usual, it will be toast pretty soon.
Diode with a limiting resistor in parallel on the input side could help a bit.