Different cylinders are rated for different pressures (and hence different gases). Not all gases remain liquid under the same pressure and thus their bottles are matched to handle (well actually exceed) the pressure of the gas they are designed to contain. eg. LPG cylinder vs CO2 cylinder.
Also, in order to help maintain the safety aspect of this (ie. not allowing bottles suitable for one type of gas being filled with another "wrong" type of gas) they all have different thread fittings so that you can't hook a CO2 bottle up to a nitrogen filling station and vice versa. This also prevents the wrong regulator being attached to the wrong bottle.
Your not going to be able to get a CO2 place to fill a scuba bottle with CO2 because it wont have the right fitting (well at least the ones I have seen haven't). I'm guessing scuba tanks are filled with a mix of nitrogen and oxygen, what pressure do that reach in a cylinder? Maybe they can fill them with CO2?
If the bottle can withstand the CO2 pressure you could possibily get a brass fitting turned up to adapt the bottle to the CO2 filling station-and don't forget to allow a CO2 regulator to screw on the bottle. Then you have to get the place to fill it and unless he was the one that offered to do it all in the first place I give you buckleys chance of getting it filled.
Your much better off going with a CO2 fire extinguisher-it's designed for CO2 and has the necessary fittings to get it filled. If you get the right fire extinguisher head your reg will screw straight on. By going the extinguisher option you'll save yourself a world of pain and endless frustration trying to adapt your scuba bottle.
Now a scuba bottle for an oxygenation system, that might be a goer
Cheers, Justin