You don't need to find a cheap paintball regulator.
Buy a normal, dual-stage regulator that's in good shape for $50-$60 and spend $10 on a CGA/paintball adapter. That's about as cheap as you can get - and it's half as spendy as the rebranded paintball-specific regulators you can buy from a few retailers.
If that works for your needs, stick to it. But many people don't use Excel because it gets expensive, can't be used with some plants or doesn't work out with sensitive livestock.
Going the regulator route with a paintball adapter may be $80-$90 up front but it allows you to upgrade to a full-size CO2 cylinder down the road if desired. There's also a ton of resale value in a regulator.