I would normally use a loopback interface to source it from but in this case, the loopback ip address cannot be announced over the g0/1 connection which is the normal route to the NTA server.
Tunnel interfaces are only used for backup but sometimes traffic come in this way, and I want to be able to see what it is.
ip flow-export source GigabitEthernet0/1
ip flow-export version 5
ip flow-export destination <dest-ip> 2055
interface GigabitEthernet0/0
ip flow ingress
ip flow egress
interface GigabitEthernet0/1
ip flow ingress
ip flow egress
interface Tunnel1
ip flow ingress
interface Tunnel2
ip flow ingress