Steve-

My sense is that the meaningful features of the slightly oversize ball are mechanical stability and gas seal.
Both of those features can be accomplished with properly lubed tight/thick patches.

Once things settle down for me (looks like that'll be once we've moved out of the state) I'll get the WR double rifle back out and run some tightly patched roundball.
The rifling profile in those barrels might end up working out very well with that kind of combination.

On 14-bore hulls, I saw a couple of things on Gunbroker.
(be patient with that site, getting spotty service from them this morning...)

Single Remington Brass 14ga Hull

WINCHESTER WESTERN 14ga shotgun ammo

Might want to contact the remington guy and plead your case - for a lower price.
The Winchester guy seems to be going for top collector market for his box of shells - see what you can do there too...

Cheers
Tinker