paintball CO2 is plain old CO2...I used to help a friend fill our 7.5 and 9 oz tanks back home when I played.
But yes, the problem is that CO2 tank is 20oz. depending on the size of your tank, you might be refilling it every month, instead of a couple times a year.
Plus, consider the cheapest standard regulator I found was $44, but by the time I got a bubble counter and needle valve, it was as much as a Milwaukee all in one, which can be had for $80 plus shipping.
If Rex says one for your non-standard tank is $150, I'd believe it. For that price you can get a Milwaukee all in one, and a 5lb or maybe 10lb tank.
There's a difference between DYI, and forcing objects to perform tasks they were never intended to perform.