I don't have a different suggestion re where to power it from, but just in case somebody else is reading this thread in the future for troubleshooting advice;
The "standard" way to wire a hobbs meter is upstream of the oil pressure switch. The hot wire starts at the buss, then goes to the hobbs with a nice short run, then out through the firewall to the oil pressure switch and then to ground.
The other way (buss - out to oil pressure -back in to hobbs - ground) takes about twice as much wire to get the same result.
Looks like the OP already solved his issue, but if it's wired like this, they simplest way to trouble shoot is to bypass the oil pressure switch- just jumper the "in" wire to the oil pressure switch to ground. If the hobbs starts working then the switch is bad. if it doesn't, then the hobbs or wiring is bad.
Hopefully it's obvious that if its' wired "wrong", with power going straight from the buss into the oil pressure switch, this is going to have a bad result, so make sure of your wiring architecture first.