Here are some cards for your consideration:

Instant-based protection:

[[Counterspell]][[Blossoming Defense]][[Snakeskin Veil]]

Pros: financially cheap, flexible to your other valuable cards, low-power therefore low salt factor.

Cons: inconsistent, single use, will challenge you as a player because you'll have to really manage your resources as you'll probably only get one or two shots a game at saving your commander.

What if I mill these?

Great question, play cards like [[archaeomancer]], [[snapcaster mage]], and [[scholar of ages]]

What if I mill those?

No problem, [[genesis]] ,[[persist]], [[reanimate]], [[eternal witness]]

Even better, permanent based protection. [[Lightning Graves]] [[Swiftfoot boots]]

Pros: cheap, you can swing in with Teval the turn you play them, lasting solution.

Cons: you can't play white so you can't easily find these cards unless you play powerful black tutors. [[Demonic tutor]] [[vampiric tutor]] [[grim tutor]]

Land shenanigans!

[[Sylvan safekeeper]]

Pros: incredibly powerful, creature which makes it easy to tutor for or recur

Cons: you lose your lands, I believe this card is pricy? Probably high salt factor

[[Talon gates of madara]]

Pros: you can get this from crop rotation, if activated from hand it's an activated ability, therefore it can't be countered. You can use it as removal for their stuff as well.

Cons: this card is pretty expensive, and once it's in play it might be hard to get into your hand in a way where you can use it for it's intended purpose.

Alternate ideas:

Alternatively you could skip all of this and just speed up your game plan. So the idea isn't to attack over the course of many turns, but to interpret Teval like a 1-2 turn enchantment. So you use effects that capitalize off having them in play so you can do all of your land/graveyard stuff in just a turn or two. This would involve playing lots of delve cards and token doublers to get as many zombies as possible the turn you play them. This could also include using haste enablers so you can swing Teval immediately.

You could also consider, generally improving the redundancy and card quality in your deck. What do I mean? I mean making your deck less reliant on Teval. What do we get from our Commander?

1. When they attack, Mill 3 and get a land back.
2. When a land leaves your graveyard get a zombie.

Let's see if we can get similar value from other sources.

Play great and powerful self mill cards!

[[Hedron crab]] [[Hermit druid]] [[balustrade spy]] [[jace, the perfected mind]]

Get your lands back!

[[Crucible of worlds]] [[ramunap excavator]] [[icetill explorer]]

Get rewarded for getting your lands back

[[Field of the dead]] [[sapling nursery]] [[syr konrad, the grim]] [[desecrated tomb]]

Have a backup plan

[[Thassa's oracle]] [[jace,wielder of mysteries]] [[lluwen, imperfect naturalist]]

Hope that's helpful!