If you want bulk legitimate Teensies cheap you talk to Robin at PJRC or your local seller
https://forum.pjrc.com/threads/23601-Official-Distributors, and expect to pay pretty much the web store price given there isn't much margin between the parts cost and the sale cost, but shipping a bulk order out the door puts only one set of handling charges on which you can haggle over if you are talking 100s.
If you want counterfeits then this is a pretty crass way of doing it.
edit - re going the counterfeits - many of them work but if you haven't met bottom of the barrel electronics before you need to think long and hard about how the following will impact whatever your project is:
No QC at factory- you will need to manually test and discard everything you buy, assume a 10% dead on arrival and factor the time to do this before assembling into your project (possibly building a test jig)
Different bootloader (but pretends to be a teensy)- Will work for some code some of the time, in particular seems to be able to overwrite itself and brick without warning
Recycled parts - many bottom end parts have had a life (and abuse) before you get them
IC failed QC - part works but failed a soak test, more likely to randomly crash - often mixed in with 'real' parts
IC old or test stock - part does not have features/work as data sheet says it does in all particulars
No corrosion protection - need to use board quickly and with more solder flux
Absent or out of spec protective components - 0ohm resistors are cheaper than PTC fuses, and it only matters when something had already gone wrong...
mixed models/designed - not all parts may be the same layout/size - complicates rest of design to allow for.
If this is a single purpose art project thing then maybe the above makes sense (can be an interesting learning experince). If this is a product you have any sort of warrenty or liability for you need to think about just what you are doing.