If it ever seems like the refill/pen is dried but you can clearly see ink still in there on a rollerball or gel pen, dip the tip in some really hot water for a few seconds and it'll revive most refills.
You'll find these on ebay or etsy a lot. Aliexpress too. I would recommend also Wingsung 601 (for the parker 51 vacumatic experience), the Wingsung 698 with a gold nib for a nice piston, Hero 100 for an aerometric parker 51... or if you want more modern designs, look for Moonman, Penbbs, Live In You brands.
Also, a plug for Wingsung! They'll look very similar to twsbi or parker 51 at times, but imo they've added some little touches that improve on the twsbi.
Assuming you're in the US, you'll be able to find a lot of pens and ink on Amazon. If you're looking to support American retailers, try Vanness Pens, Anderson Pens, Goulet Pens.
Huuuge communities online exist though. You might be surprised - check out fpgeeks forums or fountain pen network - both forums list local gatherings and pen shows.
And there are pen shows around the world where collectors and enthusiasts come together. e.g. sf pen show in the bay area (sfpenshow.com), LA Pen show (http://www.lainternationalpenshow.com/) which as been going for like 30 years, and more - many more.
Platinum 3776 are good and designed for this use case - the inner cap is supposed to keep inks from drying out even with long periods of no usage.
If you buy a restored Parker 51, they're nearly indestructible. Just dip it in a little water to get it flowing again.
There's a HUGE market. In fact the Japanese market alone is large enough that Pilot/Sailor/Platinum don't even release all their new pens to the US market.
Try going to the forums - fountainpennetwork.com or fpgeeks.com to get interest in your blotters. I've seen rocking blotters sold by J. Herbin, so they still exist.