So the following are top notch and part of TacComs
SCP Exodus Wars
DRM
Onslaught Miniatures
Trouble Maker Games Defeat in Detail
I can personally vouch for CnC Solar Empire Marines and Microwotld Games. Lots of people use all the above
Sentinels are a harder item to proxy as is. They are pretty common on eBay still so there is that. However the rules for EA are nice due the abstraction they represent. For instance, it's perfectly plausible to use say a lightweight scout or ATV vehicle as a count-as for sentinels. The real important bit is that it's immediately clear (or easy to know for your opponent when you play) that these are such and such.
In example, I sold all my salamander scout vehicles years ago. However I need scout vehicles when I happen to use the Tank Legion list for my guard collection. Instead I use a small light weight vehicle that looks a whole lot like a centaur for that. What's nice is that I can also use those same models as actual Centaurs when I decide to play DKoK with my guards instead. Make sense? I find it really stretches my hobby dollar.
So some ideas are to find a atv, like DRM has, and say this is functionally equivalent to a Sentinel for my particular army in the battlefield. That's actually nice as that model could stand in for tarros scout vehicles so you can play Elysian Drop Troops as well and only have to buy/paint one model for both.
The Microworld and Solar Empire Marines lines both have some mechs that can work if you're dead set in having actual walkers on legs.
Another plausible approach is to say that your guard comes from a planet that has close ties to the Ad Mech. As such they actually have access to specialized scouting robots that take the role that would normally be filled by Sentinels.
And of course flea bay if money is less important than original models.
While the lines above don't have chicken walkers today, I don't think it'll be too long before someone gets something that works. So you can play the waiting game easily.
If you have specific armies/units you want recommendations on just ask