(Because right now Pirates give you the stealth missions, but you are forced to get a Tri-Tach-commission to get the stealth-ships, aka phase-ships.)
One of the common stealth missions (spy satellite deployment) seems like it often comes from non-pirate factions, actually - that mission type can give you a faction military contact - and while taking a Tri-Tachyon commission might be the easiest way to gain access to phase ships, you certainly aren't forced into it if you want to build a phase fleet. Pirate variants of the Shade and Afflictor are in my experience reasonably common on the black and sometimes open markets of non-Tri-Tachyon worlds while base-variant Shades, Afflictors, and Harbingers show up on the black and occasionally open markets of Tri-Tachyon worlds often enough that they (particularly the frigates) are not especially difficult to acquire; Phantoms, Revenants, and Dooms are a bit harder to find, but can sometimes be purchased from the arms dealers you can find while visiting bars or through your contacts, and can also be found as derelicts or blueprints in the fringe; I have also occasionally found Revenants adrift in hyperspace in the Core, usually near Hybrasil.
I started my current game by selling my starter fleet, buying a Shade (base variant, if I recall correctly) off Jangala's black market, and fitting it with expanded cargo holds for very definitely legal trade, honest, then proceeded to add five or six more phase frigates (a mix of base and pirate variant Shades and Afflictors) to the fleet when I saw them and had funds to spare, ordered a Phantom from an arms dealer I met in a bar partly because I was hoping to create a contact who I could order more phase ships from in the future (didn't work out, unfortunately), bought a Harbinger when I saw one show up on the Eochu Bres black market, and acquired two or three more Phantoms, five Revenants, and a Doom blueprint after I started exploring outside of the core; I've also engaged several pirate or bounty fleets which had phase ships (usually frigates, though one had a Revenant) that could be recovered post-battle, though I haven't actually bothered to pick any of those up. At no point in this game have I ever accepted a Tri-Tachyon commission or purchased a phase ship from a military (whether Tri-Tachyon or any other) market (I did, however, accept the Hegemony's chit to cover operating expenses while exploring; turns out putting away my phase frigates and running around the fringe with a battleship, eight cruisers, five Atlases, a Prometheus, a Harbinger, four Salvage Rigs, and enough spare crew to man anything interesting that I might want to recover isn't cheap, go figure).