iptables block access to port 8000 except from IP address

This question should be on Server Fault. Nevertheless, the following should do the trick, assuming you’re talking about TCP and the IP you want to allow is 1.2.3.4:

iptables -A INPUT -p tcp --dport 8000 -s 1.2.3.4 -j ACCEPT
iptables -A INPUT -p tcp --dport 8000 -j DROP