How to Buy?
There are four major ways to purchase cards: singles, sealed product, card lots, and complete sets. Each of these represents a different time and money investment, so pick the style that fits your available resources.
Anywhere that cards can be found, individual cards can usually be purchased. This isn't a cost-effective method for bulk commons and uncommons, but if rounding out a set collection or hunting for rares, this is often the only reliable way available.
One of the best bang-for-buck methods is to purchase complete sets, which usually means 1 of each card from a single set. Occasionally (especially for later sets) you can find partial sets, such as a "complete Commons/Uncommons" set, which is the same thing restricted to the listed rarity.
Booster packs (11 random cards) come in boxes of 36, and starter decks (59 fixed cards, 3 random rares) come in boxes of 12. These have started to become scarce, so you may pay a premium for the opportunity of opening up fresh packs. Nonetheless, there's always a chance that you score big and find a rare card that effectively pays for the box itself. The rest of the time you'll of course get trash that you could have purchased individually for less, so gamble responsibly.
Unlike some other CCGs, the LotR-TCG doesn't have as rampant a problem with pack-weighing (foil cards are heavier than standard cards, so if you put a pack on a precise scale you can reliably identify packs with foils). Nonetheless, realize that any loose packs you purchase could have been weighed already. If you're not after foils then this is a moot point.
Most likely to show up on eBay, card lots are when people go "here's a big ol' box of cards, not sure what's in it" and you take a chance on it being good. This is a very effective way of boosting your collection if just starting out, and is a gamble if you don't have big gaps. Usually such sellers don't know anything about the game, which can cut both ways (maybe it's a meticulously complete set, maybe it's actually Middle Earth CCG cards listed incorrectly).
You may also come across actual fans parting with their collection for one reason or another, and those are less likely to be as cost-effective but less obscure in what they contain. If you have the cash, such a purchase could single-handedly get you everything you want.