Me: I doubt the Manei Domini would cache significant numbers of their front line machines as they were getting wiped out.
I've told the players that if anything they might find some interesting standard, non-omni mechs in a Blakist cache, but that celestials are probably not that believable.
To be fair they're willing to pay the points required to get omnis at the weight classes they want, but I've told them that early production omnis (Black Hawk-KU, Avatar, etc.) are much more believable.
Am I being unreasonable?
I'd have to say that you're not being totally unreasonable - although there were more Mechs than MD towards the end of the Jihad, there weren't necessarily a lot of Celestials lying around despite Precentor-Martial St. Jamais' creation of the
Eidolon from spare parts... or at least, not available in places that your playesr would have easy access to, especially not since the MD took great pains to keep their superior machines out of the hands of heathens.
Plus, you have to remember that MD vehicles were, for the most part, driven by VDNI - your players aren't likely to have the implants to use them, and if they came out of the factory that way it may be hard to refit them with a regular cockpit, at least with the cash they're likely to have in hand. On top of such a cache probably being rigged with a nuclear-based self-destruct device which would be good for at least ten years... or at least, you could pretend such existed.
Remember too that mercs got a
VERY bad name from some bad apples working with the WoBblies. Being a merc with a Celestial, which was a signature WoB mech, could cause some... unfortunate situations to occur, or make potential employers immediately drop their offers and walk away due to the stigma attached. To risk the invoking of Godwin's Law, think of how people react to former Nazi concentration-camp guards even now, sixty years after World War 2. Now imagine how people would react to a merc band which shows up sporting some Celestials, and people KNOW only the WoB had those (the Republic of the Stars is VERY quietly deploying these, due to these same image issues).
While it might be fun to have them get their Celestials, then send them fleeing for their lives while Blake-hunting bounty hunters go after them due to a case of mistaken identity or having them suddenly face-to-face with a RotS recruiter suggesting that they work for them lest there be... fatalities due to an angry populace, unless you're Wombat, I'd have to steer you clear of that.