Next time you're trolling your local flea market/antique shop or eBay, look for an RCA Wearever set. They're in plentiful supply, typically sold at antique shops and such as a two-ballpoint/one fountain pen set. They're extremely inexpensive, but the semi-flex nib on the fountain pen is excellent!
Here's what to look for.

You'll want to, if possible, find a set that includes the cartridge. They only take Wearever cartridges, which are NOT in plentiful supply. Take the cartridge, flush it out, and then fill it with your choice of inks using a small syringe.
If you can't find a set with a cartridge, get some silicone grease and convert it to an eyedropper fill, and then get to writin'!
They typically came with only a medium nib, but it is lot of fun. It's a stainless steel semi-flex that gives some very nice line variation. If you're anything like me, you'll find that your writing has a ton more character than ever before.
And...if you can't find a set, send me a back channel message. I'm pretty sure I still know where to find one in Des Moines and I'd be happy to pick it up for you.
*Edited: Picture borrowed from Dennis Lively. I trust he wouldn't mind!