If you think about it, leather straps are in great demand for crafting. They cannot be harvested as many reagents can be. They can be transmuted with only 5 bones, but bones aren't harvested anywhere in the spiral either. Believe it or not, leather straps are used in 287 recipes! Here's how it breaks down:
|68 Pairs of boots|
|66 Housing Items|
|Both pogo stick mounts|
|Ninja Pigs Spell|
|15 Treasure Cards|
Luckily they aren't quite as hard to come by as braided vine or amber dust. In fact, 271 creatures are known to drop leather straps. Hunter Windbreaker can help you out with 72 creatures that drop leather straps. I really wish he would get us a complete list one of these days. Sometimes I suspect that I've done more work in his area than he has.
Farming for something as mundane as leather straps can be horribly boring. It is best, of course, to look at what other things the creatures drop. Then, instead of being disappointed that Simon the Sayer didn't drop that Polterpooch, you might be happy to find that you were rewarded with the leather straps that you need to craft the Regular Keeper Aquarium that you've been gathering ingredients for.
Personally, I prefer to garden for reagents. Bet you didn't see that coming huh? Only 2 plants will possibly provide leather straps, Orange Dandelions and Trumpet Vines. I have grown 10 of both of these plants for 30 days. Orange Dandelions coughed up 9 leather straps where Trumpet Vines rewarded me with 35 Leather straps over the course of the month that I grew them. That's an average of at least one per day of effort.
So in a nutshell, if you are in need of leather straps I would recommend big wizards try growing Trumpet Vines. But fair warning, they will get rank 5 pests. Orange Dandelions, on the other hand, get rank 4 pests but you can fit 10 plants into a small enough space to get away with only a medium spell to take care of their other needs. This will require more patience but works well for smaller wizards who don't have a lot of energy for gardening. Also, I harvested 10 bones from Orange Dandelions in the month that I grew them, which can be transmuted into 2 more leather straps. If you have the energy for it, you can plant more Orange Dandelions within a large area than you can Trumpet Vines, but seeds are going to be a problem.
The downside of growing Orange Dandelions is that you most likely won't get your seeds back to replant. You can buy more from Marley in Jade Palace, Mooshu for 1200 gold each. That can really add up quickly. Trumpet Vines can't be purchased until you have access to Harley in Celestia and they cost 2400 gold each. Trumpet Vines are weeds though, you can start with just a couple of seeds and have a full large plot of them growing in no time. Both plants can be auctioned which means it might be possible to find either of them in the bazaar.