There is a special run section on the Remington website.
But they also reportedly do runs that don't get catalogued.
Some specials are distributor only, so I'd check Davidsons and other distributors/wholesalers for what they might have.
I dunno if they make a youth model in polish now (haven't checked).
Current youth models (Express) run a reg length mag tube. So standard LW bbls will fit.
Special Fields are sweet, older and better built, come with 21 or 23" bbls..........but those bbls don't swap with regular 870's. Special Fields run a shorter mag tube, so with 870 versions you'll run only Special Field bbls.
If $ is not a problem, get an old Special Field 20 gauge, and swap in a youth wood set. But you're hosed on getting a longer than 23" vent rib bbl.
The newer Express models seem to be a bit iffy on quality (Cerberus era guns) older youth Express models can be fine. Get one of those and a deer bbl or longer reg one will be easier to find (compared to a Special Field).
New Remington bbl prices are scary.
You could always just get a walnut/polish LW 20 gauge and buy a synthetic youth set, or cut the reg walnut buttstock and have a pad put on at youth dim. I've seen some older RKW finish mid 80's/early 90's LW mags with Remchoke go for $350 clean. Some shops charge around $80 to fit a good pad. So for a better gun like new you'll be just over $400, maybe score a better deal than that.
You won't touch a Special field for that, not on the current market. Expect almost twice that for a minty one