Hello Vinod,
you mentioned that you currently work as a SAP support expert, this means that you have access to read SAP notes, which is of a great help, believe me.
The other thing you can use to your advantage during preparation is that most probably your company has a IDES system or a sandbox installed, where you can try out some of the things you will learn. If you haven't done so - be proactive and ask for such access. If you can explain to your bosses that you are doing this to improve your knowledge and performance, they might help you out (provided you do that in your spare time, after working hours). It depends on the personality of your boss, sometimes this is not a wise move.
You probably work in a big team - maybe you can ask your colleagues and borrow some books - SAP Press can be expensive for the standards in some countries. It is definitely possible to prepare for associate certification if you are motivated, read and practice daily for several months.
Going for trainings (unless you register for online courses) can mean taking absence from your job, which is not always possible.
You say that you have average knowledge of SAP, you will need to boost your confidence a bit before attempting certification. Even if you are not assigned to an incident/change request for SD, read it and think how you can do it (test your idea in a sandbox system), then read what the other consultant/architect proposed, try their solution and compare to what you would propose - this is how I started learning SD without much of a guidance; it worked for me - it might work for you.
Also doing certifications with my experience is right move ?
If you apply for associate level after these 5+ years support - I think yes, for professional level it can be tough - depends.. if you only work on incidents now or you also design solutions for SAP change requests/projects.
Am not going to search job outside once I done my successful certification planning to move internally within company is this right plan?
Seems like a nice way to boost your career (but it really depends on how much certification and/or superior knowledge of standard SAP is valued in your company and what are the options to switch to a different role).