owb for a glock 30

I use Bianchi size 14a but I like retention straps. Look into Safariland polymer holsters with lining. Looked over basic plastic holster with belt loops and no retention straps for G19, G22 and it looked pretty flat and easy conceal setup. Glocks look better in plastic holsters.

http://www.lobogunleather.com/

I carry my G30 in a Lobo snap on/off holster but they also make a nice three slot pancake that would work well. He does good work for a good price and you have a choice of colors and options.

Good Luck!

I use Bianchi Accumold Mod. 7506 size 14a. It's reinforced fabric not leather but it works very well for me. It rides high, does not cant and I like convenience of dual snap belt attachment very much.

On those nylon holsters and poorly formed leather holsters you really need a retention strap since the only retention other than the strap is really just gravity. Conversely, on a well made leather or kydex holster, retention is provided by the quality construction of the holster body itself.

Lobo Gun Leather, as mentioned earlier, has a good reputation and their products are relatively inexpensive.

Simply Rugged "Cuda" is an often recommended pancake design.
http://shop.simplyrugged.com/ecommerce/Cuda-.cfm?item_id=155&parent=669

I know you asked for leather, but kydex always seems like such a natural for Glocks. A kydex pancake like the Raven Concealment Phantom (or any of the many copiers) would be a good option.
http://www.ravenconcealment.com/