Ask for the denominations you want. Most dealers in Las Vegas will even pick out some in better condition if you ask. Many casino cashiers will do the same. I can't recall having a Nevada cage cashier refuse to sell me chips, although I know some in other states, particularly Indian casinos, have that policy.

If the chips are really old and worn, you can buy a rack at the cage and pick out those you like, selling back the rest. This isn't really necessary though if the chips in play are in the condition you normally collect.