I Need To Get More Cards For trading

How old are we talking here?  Most game stores sell out of print booster boxes, they cost a bit more but you'll get a ton of older cards that way.  Only other suggestion is to find someone who's getting out of the game and buy their collection.  Again, it can be costly, but in the long run it's still infinitely cheaper than buying packs.

There is a fourth way: play events and win packs; use those packs to trade for what you want.

If you do decide to buy some, ebay is perhaps your best bet--if you are patient and buy only on the cheap.  In the last two days, I got a Warren Instigator (NM) for $4 (including shipping).  I also recently picked up quantities of each Goblin Lackey and Goblin Sharpshooter via ebay.  My cost per card (shipped) was at or below half the TCGPlayer average for them.  I could turn around and sell/trade them as singles for twice what I paid for them (if I wanted to part with them).  This is especially true of the $4-5 cards.  You can often pick up playsets of four for $2-3.  Turn around and trade those, and then keep trading up. A few months back, I traded up from about $12 of cards (purchase price) I got on ebay to around $85 of chase cards and shocklands.  It took me a few weeks to do it, so that was an additional cost, but I was in no hurry, so it didn't hurt.

Of course, to do that, you need to make sure you are buying multiples (x3, x4, or larger lots) at the last minute.  I try to focus on some trusted sellers (e.g., Kid Icarus) who combine shipping.  When I have the cash to do so, I search ebay for "MTG" and a card name, and always sort by "Ending soonest". You don't always win (others are trying to snipe, too), but it has been a strategy that works well for me.

Best of luck to you!

There is a fifth way - yard, garage, and tag sales.

Chances are slim, but I've heard multiple people in my LGS talk of finding binders of cards with gems in them for cheap, cheap, cheap.  Not always MTG cards either, but some of the other TCG's out there as well.