I try to restrict purchases to just what shows up in the unit market because the additional challenge and randomization is more fun- but I was also able to come up with a home-brew system for making specific purchases and/or bulk orders.
Modifiers for purchasing include basics like the unit's current MRBC rating, tech level and type of machine being sought,
Also more detailed information like current location, how many additional C-bills are being spent on the search, whether the mech or vee is still being produced.
And then finally the more nuanced stuff like unit quirks (good/bad rep, easy to pilot, good life support, etc...) scrounge skill of the admin who got tasked with tracking down a used ASN-21 on FedSuns.Craigslist.org because the tech team botched their maintenance roll and then presumably spilled yet another Big Gulp into the cockpit of the one machine whose movement profile you built the whole recon lance off of.
Being able to place bulk orders has been great too. That way, if your XO knows a guy at Earthworks Inc. You can spring for a lance worth of GRF-1S, fresh off the line (big feature given the quality of my techs, man those guys will break anything!) and then spread them out among the unit and use them to fill out your TO&E after the Marik Militia managed to somehow hand you 70 percent combat losses during what should have been a simple smash and grab. A bulk order or two and then whatever shows up in the marketplace while waiting for them to ship makes rebuilding and reorganizing (something else my unit usually spends a lot of time on) a great deal easier.
Been tinkering with the tables here and there for months, but getting the basic modifiers down on paper or plugging them into a spreadsheet for their own purchasing system is something any reasonably interested player should be able to knock out in an hour or two.