If you don't find a seller, it takes a bit more legwork, but you can get those kinds of numbers fairly cheaply in Ilshenar - provided no one else gets there first.
Go to the UO Stratics main page, and look for the info on the randomly spawning Gypsy camps. there's typically 3-5 of them active at any given time, and 1-4 of them will have an iron worker (smith) and/or Vagabond (tinker), or 1-6 NPCs with ingots. If no one has yet found the camp since the spawn, the ingots start at 8 per, so if you buy 5k from each, you'll average 10 per ingot.
The downside is that 3 or 4 of the areas are in dangerous areas (would you consider multiple juka, or 4 water eles+lots of imps, more dangerous) - at least, dangerous areas to be shopping. A couple more are dangerous only if you're negative karma.
I'd be willing to venture on high-population shards like Atlantic & Chessy they'll be hard to get to first, so you're better off trying to buy from another player, but keep the option open for emergencies.
