It is highly unlikely that the Rebels had specialized craft manufactured, and were merely buying what was already on the market, or rather using the leftovers of older military forces (i.e. Y-Wings).
And yet, that's what the back story of it was, in what is now "Legends".
"To design the starfighter, Dodonna turned to Walex Blissex, famed ex-Kuat Systems Engineering engineer. Blissex brought valuable experience from work on the Clone Wars-era Delta-7 Aetherspite-class light interceptor and Alpha-3 interceptors. The pair based initial designs on the R-22 Spearhead, already nicknamed by pilots as "A-wings" per the -wing naming convention for other Rebel craft, of which two had fought at Yavin. They formed Alliance Underground Engineering,[15] and worked with pilot Jake Farrell[18] in improving a Tammuz-an design.[15] It was additionally based off the Delta-7.[1]
Dodonna and Blissex presented a proposal to Chief of State Mon Mothma[source?] for official support. It was ill-timed. The victory at Yavin incited open rebellion on thousands of worlds, resulting in widespread Imperial suppression. The Alliance was scattered, and had insufficient funds for the proposed A-wing program. Nevertheless, Dodonna's fame from his role at Yavin made a denial difficult, and Mothma approved with reduced funding.
The reduced budget forced Dodonna and Blissex to substantially modify the original design. Blissex redrafted to use components readily available from Ordnance and Supply Command, pushing each beyond factory specifications.
The earliest A-wings were hand-assembled at Alliance facilities like Cardooine and Chardaan Shipyards. This resulted in interesting modifications; some fighters incorporated actual wood furnishings for the cockpit interior, such as the one that Rogue Squadron pilot Tycho Celchu flew during the Battle of Endor.[8] In 0 ABY, the Soaring Dactillionss were assigned the prototypes to fly at the Cardooine Rebel base. After a week, they came under attack by the Empire, and the Dactillions flew the fighter in battle. Although they proved that the starfighter had superb qualities, only two of the pilots survived.[20] "
Not built completely from scratch, they used a lot of off-the-shelf components (much like the F-117's development), and took inspiration from a bunch of different, older ships, but before the new canon, A-wings were designed and built by the Rebel Alliance, not one of the established manufacturers.
All that said, I do find the new canon origins of the A-Wing more credible and satisfactory, although I'm not thrilled that (if the write up in wookiepedia is accurate) they've changed the specs to make it so the Rebel A-Wings have had their shields removed, when previously the A-Wings in the RA fleet did have shielding.
