Where can you buy pest strips?

They have pretty much faded away for flies since they are unsightly and Health Depts. don't like them. There are far better solutions, You can spray or bait or use fly bags or machines outdoors for houseflies/flying insects and fly lights for indoors are best for any that do make it inside.

Exterior-

Flybags-effective, but unsightly, most you just add some water and hang.

Maxforce fly bait-a scatter bait for enclosed areas like dumpsters

Spray-Maxforce fly spot bait-Bait you can spray on surfaces, works just as well and is invisible.

If you want an adhesive trap you can discard, search goldsticks and buy the long ones, but they fill up fast.

For machines any outdoor will work, for indoors use a light trap only with no grid (doesn't zap, you don't want it popping inside) This is what commercial facilities now use.