I have zero knowledge of electrics and buy the bits I need then cobble them together. The thought of a “spectacular” failure with a relay or similar makes me twitch a little.
Another good choice can be hobbyist or educational sites that have some reputation to uphold, the most talked about online will be US centric like SparkFun or Adafruit but there's many European equivalents for example. One way to find them would be to find an authorised distributor in your country for larger projects like the Raspberry Pi and see if they sell the components you need. Chances are if they're an authorised distributor for a recognisable product then they're probably not going to risk losing their spot on that products website just to sell some fakes or dodgy components.
The key thing is to not buy directly from China or random eBay sellers from abroad if you can help it, return postage to China is prohibitively expensive and Chinese stores or marketplaces like Aliexpress won't always agree that you're entitled to a full refund without returning the item even if that item happens to blown up in your face, and likewise you can't really expect that small random foreign eBay sellers are going to be liable for selling you items that adhere to your countries safety regulations for instance.
In the UK we have both CPC (~.farnell.com) and Farnell (uk.~.farnell.com; née Element14).
It's not always clear to me which I should be using (optimally for me - to them I think it's clear that CPC is trade/savvy DIY, and E14 is larger electronics buyers) - one or the other can be cheaper on the same product.
Recent examples - that sort of make sense given demographics I mentioned - small volume components cheaper at CPC; (low-end/budget) test equipment cheaper at E14.
For a few extra bucks you can use a programmable temperature controller... search "Programmable Temperature Controller Ramp Soak Heat" at the usual suspects. Allows you to program heating ramp up, ramp down, and soak times - very useful for heat treating various alloys.
They also make some very low cost PLCs that work pretty well. I built a solar powered DC irrigation pump with proper PID control+SSR to support varying flow rates(most irrigation pumps are on/off only) with their hardware.
With a PLC, analog input for the pressure sensor and SSR to drive the PWM it was fairly straightforward to put together. Hardest part was getting the PID tuning right but even that only took a bit of time.
If you're building one of these, you might want to adjust the hinge so that when you open the oven doors, the hot side of the oven door always faces away from you. It will make it much more comfortable to use.
For example, without a heat treatment oven, it would be very hard to make a new hard cutting tool. What would you cut it with? You basically have only the option of grinding it, which is very difficult especially because if you overheat it at any time you'll destroy the temper.
But if you soften it, you can use other steel tools to sculpt it to any shape you like, and then you can harden it again!
Now you can make bearings, hinges, chisels, files, sawblades, knives, wear surfaces, deadbolts, springs, needles, gears, and all sorts of other things that require a hard material in order to perform or last a long time.
I suppose though if you can get it to hold cooler temps too, then it's just an oven, could use for reflowing solder, testing performance of something at higher temperatures, warming bagels..
The video description has all of the parts.