Some depends how much money you have.
Basically buy bits off Ebay, especially when someone's offloading a bunch. Keep an eye open for 'em.
Buy the
GW kits new, like the Trukk, Battlewagon, Mek Gunz (also load of extra stuff), and the plane.
Get bits from friends and/or models - once they know you're looking for stuff to orkify, they'll probably keep you in mind especially when getting rid of old stuff.
Other places - try looking through the links at the top of the forum. Personally I love Kromlech - both whole and bits. There's a couple others but I think they're in the aforementioned thread and I can't recall them off the top of my head.
I love
https://www.bitsandkits.co.uk/ for
GW Bits - great when you're looking especially for a particular bit. High turnover, but also pretty fast restock. And he does have an alert/wishlist type system which is handy.
You can also buy models off Ebay to cannibalize. Deffkoptas (the Assault on Black Reach ones) are *awesome* for all kinds of conversions, besides just being a cool model themselves.
Plasticard:
amazon sells some (fairly limited though I've found)
If you're near a HobbyTown, they carry a lot.
Scenic Express carries some.http://www.sceneryexpress.com/Scratch-Build-Materials/products/1223/
You can try Evergreen
https://evergreenscalemodels.com/ (one of makers of styrene stuffs)
Gale Force Nine also carries some (though I think it's a little more expensive than others)
basically look up styrene in your area, scratch building, railroad modeling companies etc - maybe see if someone has a local distributor. Or check if there's a hobby/model railroad store near you.