If you can afford the luxury of possessing CounterStrike, you can afford to purchase HalfLife as well. Its as simple as that. Theres little point living in the developed world, using technology such as a computer that can run CS, and then trying to convince us over the internet that youre too poor or unimaginative to buy a $10 game (a game, I might add, that has more replay value than any game Ive bought for $90)
If you havent already, get a job and a bankaccount. Sign up for a PayPal account and link it to your bank account. It will take a couple of days to authenticate, but once youre set up it will be a godsend for you. The only bad thing about this is it can take a few days for money to transfer between your account and your PayPal account. I recommend using something like a debit Mastercard - its a normal debit bankcard but can be used as a credit card also. Since I set my PayPal account to my debit Mastercard, it thinks its a credit card, even though anything I spend is taken directly from a bank account. So ask your bank about debit Mastercards, just in general they are totally handy.