Well, it's a sort of similar set up to Skyrim : The best spells in general, including conjuration, can be bought from the magical institution ( Although in this game, that is the mages guild) . You also don't have to join the guild to get most of the spells, excluding the arcane university.
Otherwise, there's a few magic shops kicking around the Imperial city, but there not exactly great, and also spell tomes, if you have it
Ah yes, another thing, you buy spells rather than spell tomes in oblivion, doesn't make such a huge difference. \And "soul trap" is part of the lost magic group "mysticism" - so if your hoping to do that, you will need to branch out a bit.
As for your claymore, in oblivion weapon skills are graded as " Blade and Blunt" rather than " One handed and two- handed" - This means a claymore will be a "blade" rather than a "two handed" - a small but important difference!!
And it's going to be a long time until you get Daedric / Ebony, although the same goes in Skyrim.