I bought my thermocouples off Ebay - I put ten around my oven in different locations and different depths into the bricks (so that I can see how much heat soak has been achieved). They weren't expensive ones and haven't yet failed after a couple of years used.
At the moment I have a temperature monitor with a twelve position switch that I got second-hand, so have to manually choose which sensor to read. I have built but not fitted a Raspberry Pi multi-channel logger (see my other post in this folder). You do need to be able to solder small IC legs, but my software (and there are others who have done similar) should mean that the hard work of working out the hardware design and coding software is done for anyone who wants to replicate mine setup.