A good start would be a Ninja 13 CUIN paintball tank. The "High Pressure" version is 800-850 PSI. You can shim the spring regulator ones up a little to about 900-1000, but if you switch it to belvue (spelling) spring washers you can push the reg up to as high as 1800 PSI. There is a very good podt over on Air Gun Home regarding "shimming Ninja Regulators". Or if you prefer to just pick one up, JDS Airman, and HPA experts both offer tanks regulated to the output of your choice. I have both a Ninja tank I adjusted myself, and an Expert HAP tank. Happy with both, if your gun is already shooting 700 with CO2 at say 70 degrees, I would at least try a standard paintball 13 cu in tank first, as that's right in line with CO2 pressure at about 65-70 degrees. Just my 2 cents.
P.S. Feel free to email me if you like. I built mine, just because I couldn't afford the great work Roy does. When I do have the funds, I would buy one of his creations in an instant!