Jesus alluded to this passage in Isaiah on the last day of the Feast of Tabernacles saying, "If anyone is thirsty, let him come to me and drink. Whoever believes in me, as the Scripture has said, streams of living water will flow from within him." John 7:37,38
So also in John 4 where Jesus spoke with the woman at the well saying, "Everyone who drinks this water will be thirsty again, but whoever drinks the water I give him will never thirst. Indeed, the water I give him will become in him a spring of water welling up to eternal life." John 4:13,14
What Jesus offers is free
Rev 22:17 The Spirit and the bride say, "Come!" And let him who hears say, "Come!" Whoever is thirsty, let him come; and whoever wishes, let him take the free gift of the water of life
And yet notice the paradox here in Isaiah where he says, "you who have no money, come, buy and eat! Come, buy wine and milk without money and without cost." For how can one buy things without money? Perhaps this is also in the same sense as in Revelation 3:18 where he speaks to the church of Laodicea saying among other things, "I counsel you to buy from Me gold refined in the fire, that you may be rich; and white garments, that you may be clothed ..." And so also in the parable of the ten virgins of Matt 25 where the five were instructed to "go to those who sell oil and buy some for yourselves." Matt 25:9 The oil most likely alluding to the Holy Spirit, or that guidance which comes from the Holy Spirit.
So how are these paid for? Apparently not with money. But neither are they free in the sense of being given unconditionally, as the term "buy" must allude to some cost. Rather the cost is simply the cost of following Jesus.
Luke 14:27-30 Anyone who does not carry his cross and follow me cannot be my disciple. Suppose one of you wants to build a tower. Will he not first sit down and estimate the cost to see if he has enough money to complete it? For if he lays the foundation and is not able to finish it, everyone who sees it will ridicule him, saying, ‘This fellow began to build and was not able to finish.’
Luke 9:23,24 "If anyone would come after me, he must deny himself and take up his cross daily and follow me. For whoever wants to save his life will lose it, but whoever loses his life for me will save it."