Trying to get mine running and there isnt a battery in it (came without one). The one from my Volvo 850 fits in it, but I am wondering what battery people usually get. Does it cross reference with other Nissan cars?

Should be the same as a 240SX.

http://www.md-battery.jp/fs/batteryichiba/c/gr13552

The terminals on the OEM battery cables are too small for most American batteries. Swapping them out to larger, USDM terminals will give you more battery options. If not, a Toyota Pruis backup battery fits perfectly. Personally, I had to swap the terminals and put in an out-sized battery in mine to drive it out of port and am swapping it with a flat top Duracell unit from Batteries Plus so that I can use my battery tie-down again.