You can substitute Maggi with instant bouillon..I use Wyler's Instant bouillon in my pot roast recipes. Maggi is a favoring..sometimes in liquid form or dry granules or cubes. Can be in chicken flavor, beef, asian and a few other flavors.
In some of the local Polish stores in my area of IL they still sell Maggi but I can't remember the brand name. I called my Mom to ask her about this and she said that Maggi is not like Worcestershire sauce, has a completely different taste and it's used in soups, sauces and other things..like pot roast. I've never used Maggi as most of my Polish recipe books ask for bouillon. Sorry I can't be of more help.
The name comes from a company which invented Maggi. Now many companies produce similar seasoning usually called Przyprawa do zupwith little difference in taste but the most popular is Winiary. If you can't find it, the closest I know is soy sauce, a mostly salty flavour but with a twist.
I've just found the thread about online deli shops for the USA, look for Polish Online Deli serving the US they should have maggi