There is a good reason that they test you for HIV first before giving you PREP. If you happen to be infected, and you start taking PREP, you can and probably will become resistant to the drug, and any other medication you might get in the future can become ineffective. Pulse clinic here in Pattaya can test you for HIV on the spot. You can also pick up HIV self test kits in most of the local pharmacies which work like the Covid 19 tests. You prick you finger, not finger your prick, and put a drop of blood into the test box. Wait a few minutes and you will have your result. Then, if negative, you can start taking PREP. The test claims to be 99% accurate. That being said, you need to wait a few weeks after your last encounter before testing, since it will take some time for antibodies to appear in your blood. Also note that Pulse clinic does not always have a doctor present on premises. They do have a schedule when the doctor is available.
It is a totally ignorant idea to purchase PREP over the counter without the proper preparation and testing. It is your life and if you want to take chances with it that you cannot reverse later, it's up to you!