OngZNI would always recommend going to a local watch shop that you trust or is popular in your city. Firstly, it's difficult to see if the colours and textures match your watch if you buy online. Secondly, it's a guaranteed fit and satisfaction when you purchase and wear it.
If you still think online is best for you. Massdrop has a few drops, look out for them as most are of top quality. Ebay has a massive affordable collection. Avoid the cheap $1-10 straps. Something above $20, calf leather is best. Also amazon has great value on branded straps. Do some searching on google and you'll find some recommended brands for leather straps.