Ok; you probably don’t want metal minis (some people prefer them, but you’d have to paint them yourself, and they are a bit fragile for my taste anyway).
So, in term of plastic minis… The “normal” way to buy them is in blind boxes, buy the box, hope for the best, not suitable for you.
However, some minis series do come in transparent boxes that do let you look at them and choose them. Both D&D and Pathfinder had such series and I have seen a few such boxes at PH. Not perfect, if only because you have to buy a handful of minis only for the two that interest you, but there is this solution.
Otherwise, and that’d be my advice, there are sites that allows you to choose individual minis to buy (when bought that way, they are called “singles”)
The prices of such minis, however, fluctuate widely, depending on the rarity of the mini you are targeting. Common stuff are generally less than a dollar, rare stuff… Well it can get a bit crazy.
And, yes, the price I gave you was in dollar, I have not bought singles since I came back from the U.S.
And that’s where the sites I know are located, which might make shipping a bit costly and, more importantly, slow.
Anyway, you can try "Miniature market " or “Troll and Toad”. Look for pathfinder or D&D but don’t neglect other fantasy producers. Rackham had a few cool ones, before the company folded, but some minis might still be on the market…
Also, I have seen people sell their miniature on Ebay, but generally by lots…