Lamy™ M21 or Schmidt™ 635 mini
Most can be used. My favorite is the Schmidt® 888. Others are Pilot® G2 gel, Hauser®, Monteverde USA®. Just be sure not to lose the spring in the barrel when changing.
All the fountain pens take a Universal Fountain Pen Cartridge and come with a converter. All will use the long Waterman® that you can get a Staples® or Office Depot® except the Havana Fountain pen. It takes only a short style cartridge and converter. Other brand name cartridges are Waterman® Pelikn® Delta® Montiverdi® (Don't use Cross® it will leak) There are many sources like www.Amazon.com, www.levenger.com, and www.YafaOutlet.com for generic ones.
With the converter, you can use any bottled Fountain pens ink. Do NOT use India ink. Try Sailor® Aurora® Lamy® Pelikan® Parker Quink® Montblanc® Levenger® Delta® etc.