I think I am stuck, gear-wise

I'm posting this to seek advice.

I am level 26. My gears are all rare/yellow, but they were acquired when I was level 8 to level 16. I feel my gear is falling behind because I'm suffering more damage now from mobs in the Lost City. While I get rare items from drops, they are rarely suitable for my Witch Minion build. Drops of orbs that can upgrade gear to rare are scarce, so I'm not optimistic about relying on them. I have never seen NPC vendors sell rare items either.

I find it challenging to upgrade gear in this game, and I fear I am reaching the point where I will lose interest in the game because of gear scarcity.

Have any of you had this experience? I'd be grateful to know how you dealt with this.

I had this experience. For better or for worse, the campaign does not necessarily give you the drops you need by going through each zone once, even full-clearing. My recommendation is to grind a bit. I can't promise it'll solve the issue but it will definitely help. Just reclear a zone you didn't hate from 2-3 levels below where you currently are at, preferably one where you feel pretty capable of beating the boss quickly.

In case any of these were missed
- Disenchant Rares to get Regal shards. Not fast, but much faster than waiting for them to drop.
- Quality of mods is more important than rarity. A blue item with two really good mods is better than a low-level yellow with mediocre ones. This is up to personal judgement.
- Get your resists as high as you can.

But yeah, my view on PoE2 so far is - when in doubt, grind for gear a bit, it's in the company name. I got destroyed my first 3 or 4 attempts on the End-of-Act 2 boss, and the difference after roughly two hours was huge.

Had that problem day 1, but ever since the drops were increased, I've been able to solve early gearing issues with a bit of grinding (opposed to heavy grinding). Check the vendor at each level up, if not for upgrades, then you can at least get socketed items you can salvage. Runes drop frequently enough, that's good to solve early res problems.

Personally, I don't use the gamba vendor before maps, but your mileage may vary.

At those early levels, I never got anything good from gambling and exalt-slamming. Save your exalts and use them to double or triple your DPS.

I don't play minion so I'm not sure what gear to look for, but below is probably an example. I'm not sure it helps, but that's what I did as a melee attacker. You mentioned your level is 26 so we can search for 27 since it is just one more level.
https://www.pathofexile.com/trade2/search/poe2/Standard/klbaj8Ws5
And please be really sure this purchase will at least double your DPS, because as I said, I don't play minions in PoE2.

Save your gold to buy white/blue items with rune sockets whenever offered by the vendors. Salvage them into artificier shards.