The traditional way to find corpses of dangerous monsters is to do some research, go to where the dangerous monsters reportedly are, and kill them yourself. Want hill giant corpses? Find out where the hill giants live, go there, learn from them where they bury their dead, kill them, and make a bunch of hill giant skeletons or zombies or whatever.
The alternative—yet no less traditional—approach is for you to become the adventure hook for the campaign's other adventuring parties: go to a town, spread the word that you'll pay good gp for monster corpses, and wait for the corpses to roll in. While this scheme may run afoul of good-aligned authorities—and, perhaps, some good-aligned adventuring parties that often view necromancers unfavorably—, this has the advantage of otherwise not putting you in danger directly and costing you only time and the corpse bounty. (Or, in the latter case, y'know, not—that is, you needn't pay the adventurers who delivered the corpses—they can likely be turned into corpses, too.)
…But there's also sympathy
Despite its problems, the spell sympathy is designed for this. While an arcane scroll of sympathy costs either 3,900 gp if purchased from a level 16 summoner or 4,500 gp if purchased from a level 15 wizard—which is about 1/3 to 1/2 the gp of a level 5 PC—, consider the scroll an investment: that scroll turns into corpses, XP, and whatever treasure's possessed by the monsters attracted by the spell!1
However, you'll have to check with the DM how the spell sympathy works exactly. The Pathfinder spell sympathy is almost a complete cut-and-paste of the D&D 3.5e's spell sympathy and has the same problems. The spell says
You cause an object or location [you'll probably want the location] to emanate magical vibrations that attract either a specific kind of intelligent creature or creatures of a particular alignment, as defined by you. The particular kind of creature to be affected must be named specifically. A creature subtype is not specific enough. Likewise, the specific alignment must be named.
Creatures of the specified kind or alignment feel elated and pleased to be in the area or desire to touch or possess the object. The compulsion to stay in the area or touch the object is overpowering. If the save is successful, the creature is released from the enchantment, but a subsequent save must be made 1d6 x 10 minutes later. If this save fails, the affected creature attempts to return to the area or object.
While the spell's range—that is, the distance away from the caster the spell's effect can be created—is close, the spell's effect can affect a location to which creatures are supposed be drawn… yet the distance from which those creatures are drawn isn't at all clear. The GM may rule that because the spell's effect affects the location, the folks who are already there now just want to stay there, contradicting the spell's description. Alternatively, the GM may rule the spell is really weak, only drawing creatures from close range to the location, despite that not being how a spell's range works. Or, even more alternatively, the GM may rule that spell's effect draws to that location every appropriate being in existence no matter their distance from the effect—which seems too much even for an 8th-level spell.
This GM humbly recommends the sympathy spell be house ruled so as to have a reasonably radius (maybe 1 mi./level like the spell commune with nature?), but ask your GM how the sympathy spell works before investing in the scroll.
1 While it's a little complicated to activate an scroll of a type of spell that's on your spell list but you can't yet cast, as long as you've the Int 18, it still only takes only making a caster level check (DC 1 + the scroll spell's caster level). Success means that the spell's cast. A natural 1 or a failure means the spell isn't cast—but, so far, the scroll's fine and the spell's still there!—and making a Wisdom ability check (DC 5… yes, just 5). Failure on that Wisdom ability check means a mishap that can erase the spell. You didn't dump Wisdom, did you?