I'll let OP share his source as well, but wanted to add Grainger, which is where I bought mine. Whoever owned my 853 before me took a grinder to the chain in order to remove it and only left me with a 2 foot length, so I was starting from zero. Bobcat didn't carry chain for the 853 anymore, and it's the Wild West, combined with how you're going to use your machine, combined with how much you're willing to spend, in figuring out replacement chain.
Tsubaki and Diamond are the best manufacturers from all that I could find. Those were the OEM suppliers, and the ANSI size for 853 chain is 100. I believe the 743 uses size 80. Bobcat OEM specs were for the HS (high strength) version, so 80HS for the 743 and 100HS for the 853.
Diamond 100HS chain is $131/foot at Motion Industries. The 853 needed 20 feet, so $2,620 plus shipping plus connectors. If I were using my 853 for business or rental purposes, no question, buy the HS.
Tsubaki 100 chain (standard, not HS) is $32/foot at Grainger, so $640 plus connectors and free shipping if you're a Farm Bureau member. Tsubaki 100 chain is made in either Japan or the US.
Generic 100 chain made in India/China can be bought for $6/foot.
Used chain is out there as well, but you sure want to know how worn the pins and bushings are (stretched) to see if it's within spec before purchasing any.
I went the Tsubaki 100 route. McMaster Carr also sells chain, but I couldn't see on their website which manufacturers they were carrying.