Alright gotcha.
Yeah 83 is great for your ambient temps, but you will need a little more for your hot spot.
To be clear, you absolutely need a thermostat controlling your UTH. Now, Pet stores will generally carry some kind of thermostat, but they are usually expensive and not very good. I would suggest ordering one online. The Hydrofarm Thermostat on Amazon is the cheapest digital thermostat but isn't the -best- one available. If you have the cash, I would go for a herpstat.
In the meantime, another heat lamp can help you keep the temps up until your thermostat arrives. If you want to save money, buy a cheap dome light fixture from Home Depot instead of paying the pet store price for a light fixture that is basically the same.
As for thermometers, I find pet store ones to be over priced. I use Accurite Weather Stations and they serve me well as an all in one unit. I've found them at Home Depot in the past, but recently I haven't seen them. I would try lowes or Walmart. Just ask where the indoor/outdoor thermometers are and someone should be able to point you in the right direction. You want the unit that has a wired probe to measure the "outside" temp and measures inside temp and humidity at the unit.