My children are looking for pet costumes for dogs and cats for this Halloween,2017.Please help me by advising me about such a out-let where I can purchase those things.

Best to wait nearer the time to see what is on trend this year..
Mind you I'm sure a helpful link will be posted soon enough.

It really depends on where you are. I live in Canada, and once Halloween is near I can find dog and cat costumes in many places. If you have a Dollarama near you you can buy cute, cheap costumes for $2 for both dogs and cats. Value Village, a thrift store in Canada, sells new costumes during Halloween, and has a great selection of dog and cat costumes, ranging in price from $6-$25.

If you want a dog or cat costume right now, check online. Since you don't mention where you live, it's impossible to tell you what's available near you.

If you are in the U.S. Petsmart and Petco have them on clearance in stores, and possibly online.