Should I buy a pulse oximeter to test myself at home if I think I have been exposed to COVID-19 and it’s hard to get tested? At what % value should I get help, and what is a high quality brand that isn’t too expensive?

No. not necessary. if you have s.o.b. call e.r. or go there.
Sure, you can. If you have severe shortness of breath, you would know it. You do not have to measure it. However, a pulse oximeter is inexpensive. If it gives peace of mind, go for it. If the number is less than 93, you should seek help.
A good idea. Even if you don't have a fever(typical fever is 100.4 F), then a decrease on a pulse oximeter ( normal is 96-98%) can be a sign of covid19. Most fitbits seem to measure this but are not as accurate as a pulse oximeter.
No. No need to buy one unless you have symptoms manifested mostly by difficulty in breathing. Then if that happens you need to go to hospital .
Not a bad idea. Normal values 95 to 100%. Would certainly call your doctor if 90 or under. Measures Oxygen saturation in the blood. Call also if shortness of breath or tight chest discomfort. Can check consumer reports for quality or recommendations from your physician.

Yes. Pulse oximeters are inexpensive and a great addition to any home medical kit.
For COVID symptoms of shortness of breath or severe unrelenting headache, check your pO2 on the oximeter. It should be above 93% on room air. If it is falling below 90% contact your doctor for guidance on what to do next.
Good luck and stay safe!
Pulse oximeter is . A good idea to buy. It will show hypoxia during disease and a good indicator to call 911 and go to the hospital. Oxygen saturation dropping less than 94-92 is the reason to use extra oxygen. All manufacturers sell pretty similar pulse oximeters.
Yes. These devices are relatively inexpensive ($25 - $200) depending on connectivity. I firmly believe these types of devices along with a whole range of other devices such as AliveCor's Kardia, Blood pressure monitors, scales, and connected thermometers (Thermo, Kinsa) can certainly provide that extra objective information to help your doctor treat you. Yes. Go ahead and buy one.