There are no plastic alternatives any more, but in the old 6th Edition Starter Box (which was something like 16 Arrer Boyz and 19 Orc Boyz w/ Additional Hand Weapons) and the 6th Edition Orc Army Battalion (the same, but with a Chariot and I think some Wolf Riders too [my boxes are buried
ATM]) there were some in relatively decent numbers.
Finding them is a bit harder (
try this link, if it works) due to the fact that now they're discontinued, but this was the only way to get Plastic Orc Arrers from within the last two (now three) editions.
If you can find a lot, all the more power too you. However, unless you can find an O&G player to part with those he already has, it may be difficult to get them in anything approaching reasonable numbers. This is a shame, as point wise if it weren't for animosity they'd be
really good for what they do at their price (6pts for a 6+ Save T4 Archer, 24" range, S4 in the first round,
and can have full command with a 2 S5 attack Champion? With a 36 big unit under 250pts?).
If you cannot find any plastic ones, don't want to buy the metal ones, but really want to use them, you can always go with conversions.