Buying Gems
My local BestBuy has gem cards for 2000 gems for $25. There’s still a big stack of them next to all the other gaming cards on the side of the PC games section.
In addition to those you’ve mentioned, GameStop sells 1200 Gem Cards for $15 (same value, gem ratio is always 80 gems per $1), and you can order 2000 Gem Cards from Amazon and have them delivered.
Well there’s always Amazon.
But the OP’s answer is no. Well unless until someone posts the steps to login (may just need the game running) and the URL of the TP. It’s the exact same interface but you might feel a bit more secure running if from your browser of choice. But I’m willing to bet ANet frown on doing that.
RIP City of Heroes
To answer the question: no, you cannot buy gems through Anet’s website. At one point in the beta they did have a web-accessible version of the gem store, but it didn’t really work and was taken down prior to launch. It might return at some point when Anet decides to actually get BLTC working out-of-game.