How can I play Dominion with 5+ players without expansions?
To play with more than 4 players, the rules for Intrigue (link - page 7) give the following guidelines:

Provinces = players x 3
Curses = (players - 1) x 10
Exhaust 4 Kingdom piles (instead of 3) to end the game. Province pile still ends the game on its own.

Of course you'll need 3 more Estates per player at the beginning of the game, but for the Coppers you'll just have fewer in the supply.
All other rules are the same as in a 3- or 4-player game.

Keep in mind that more players can make a game feel slower, and can make some cards (especially attack cards) more powerful than in a smaller game. Also, Intrigue is the only "expansion" that comes with the common (Copper, Silver, Gold, Estate, Duchy, Province, Curse) cards, but once you have it you can use those to boost your supply. Until then I recommend using a few unused Kingdom cards to fill in the blanks. Between the Base set, Seaside and Alchemy I found I had at least fifteen blank cards, which my wife lovingly inscribed with appropriate verbiage to reduce our need for unused Kingdom cards.

EDIT: For awhile now Rio Grande Games has had a set of just the Base Cards for sale.