Regarding Nato Straps:
I used to buy these Nato straps from a seller on eBay India named "durashop". The straps he sold were good quality and almost all the one's I got ended up being comfortable to wear. Unfortunately though, he has stopped dealing on eBay and doesn't sell straps any longer.
I recently ordered around 12 straps from AliExpress and expecting them to arrive later this month. I'll post photographs of them here once I have them. You can search "nato straps" on AliExpress and you'll get plenty of results. Just make sure you read the user reviews of the seller and know the lug width of your watch before ordering. Two simple gold standard rules before ordering.
I would advice to stay away from the Nato straps available on Amazon India. I have bought a couple of them and they are horrible quality. The nylon they are made with is a darn joke at best.
Regarding Steel Bracelets:
I usually keep most of the steel bracelets that come with a watch (stock) unless they are uncomfortable to wear or are downright cheaply built. If I change them then I usually go with Oyster styled bracelets from MiLTAT. They are 316L solidly constructed stainless steel bracelets and are super comfortable to wear, even for long hours at work or long haul flights when traveling. Sadly these are not available in India and I bought mine from Amazon US. I don't wear Jubilee styled bracelets so can't comment on those.
Yet another option for steel bracelets (both Oyster and Jubilee styled) is from a seller named "carlywet" (just search "carlywet" on AliExpress and it'll lead you to her products). She sells a large plethora of good quality straps and bracelets , including the famous Rolex Glidelock syled Oyster bracelet which has garnered quite the good review from what I have read.
Regarding Leather Straps:
The one's on eBay India aren't good quality and generally very stiff. I can't stand stiff leather straps. I'm just a little picky regarding that. If you want to try leather straps from Amazon India then try a strap from a brand called "Kolet". These are rather economically priced straps but the lovely thing is these are soft and with daily wear grip your wrists better. But be prepared to change these at least once a year if used on a daily driver watch.
The other option that exists is getting leather straps from Hadley-Roma. They have ton of options to choose from and are smartly priced as well. These are not available in India AFAIK but sold abundantly on Amazon US. Very decent quality straps that should last you well over the years.