P0135 Mitsubishi - Space Star 2002

Question

Hi,

I keep getting the same code (P0135) on my Mitsubishi Space start despite clearing it.

Do I need to do other checks as well such as fuse etc. or should I just get the o2 sensor replaced?

May be it is asking for a lot but where would be the best place to buy the o2 sensor?

Thank you for your help in advance.