Most single stack mags will conceal well (i.e. not stick outward from your body) under a t-shirt. Some mag pouches vary in how low they sit on your belt. OWB, concealment styles tend to ride higher, and comp versions slightly lower.
Depending how you carry your gun, it might be a lot to stick in your pants but the only thing I can think of are IWB mag pouches. Singles are easier to find but they make doubles too (i.e. CrossBreed, TT Gunleather, Galco, JR Customs, Arratoonian, Andrews Custom Leather...the list goes on).
There are also pocket mag holders, but they're generally only singles.
Cheapest way is probably an old cell phone pouch or multitool holder.