What size of envelopes available in the market please suggest and where i get the good quality envelopes and office stationeries , just gice me the address or link to shop online, thanks.

You need to look it up for your area or geographic region. Stationary is not globally universal, if you are in Asia and someone sends you to a US source your printer will be throwing misfeed errors all day long due to the wrong size paper and envelopes. So at the risk of being rude This is one of those questions where someone needs to tell you to stop being so damned lazy and look it up for yourself. you were able to find this site you can do a simple websearch for OFFICE STATIONARY and insert your location so you know where to go and get it. Not EVERYTHING has to be purchased online!