I make a very special dish during the holidays and i need a LOT of different meats and i go to Harmony Farms in La Crescenta.
They have the best prices(for LA) great selection and will accomodate almost any orders.
Yes it's a bit far, but well worth it.
I've seen guinea fowl at Puritan Poultry in the Farmer's Market, though it's always the little baby ones, not the mature birds I want. I'm seriously disappointed at the general scarcity of capons here - I could always find them in Nashville, though they were frozen. Harmony Farms has neither of those in their website's list, though they might be available on order.
Superior Poultry in downtown Chinatown has guineas for $18 and pheasants for $13 (they're small and since it's a live poultry market they don't hang it so the flavor is a little undeveloped). It's my go to place for poultry - cheap and reasonably flavorful. They also have partridges, roosters (for authentic coq au vin), and wild duck in season. I think their address is 750 N. Broadway, LA