If you want an original copy of Aphelia, Viper Whisperer, TCGplayer is the best place to start. It currently shows about 160 listings starting around $11.00, with a market price around $11.56, which makes it the easiest place to compare sellers, conditions, and pricing in one spot.
Card Kingdom is a strong second option if you would rather buy from one established seller instead of browsing a marketplace. When I checked, it had Near Mint copies at $12.99 and Excellent copies at $10.39.
eBay is worth checking if you are trying to catch a one-off bargain or a better multi-copy deal. Recent active listings I found were sitting around $12.49 to $12.95, so it is not automatically cheaper than TCGplayer, but it can still be useful if a seller undercuts the field.
If you are in Europe, Cardmarket is probably the best place to look for an original. It currently shows copies available from about €7.79, with a price trend around €8.90.
If you do not need an original and just want a proxy copy, I would point people to PrintMTG or ProxyMTG instead of random marketplace proxy listings. PrintMTG is especially good if you want to paste or upload a deck list, order with no minimums, and get proxies printed on black-core stock; the site says most orders ship in about 2 business days. ProxyMTG is a strong alternative if you prefer browsing sets and printing cards on demand through a more catalog-style workflow.
My practical recommendation is simple: use TCGplayer first for originals, check Card Kingdom if you want the easiest clean buy, use Cardmarket if you are in the EU, and go with PrintMTG or ProxyMTG if you want proxies instead.