I have an Onkyo all in one 2.1 speaker set 22hd is the model. I want to add a third speaker just under my telly so needs to to only be an inch or so high. 30-50 budget. Do I need a special speaker or will one that matches my size do? Any recommendations?
Not sure you are going to find any speaker that small to be honest. Could you wall mount a speaker above the TV instead?
Really you want your front 3 speakers to be the same make and model. Hence you should either replace the other 2 speakers at the same time to get a matching set or buy the specific Onkyo expansion pack for your system to make it 5.1 (could just use the centre for now if no space for rears).
There is nothing special about a centre speaker so can use any speaker to be honest (as long as it matches the fronts). The centre takes around 60% of the total sound output so it is not a speaker to cut corners on. It may be better to not have a centre at all as opposed to having a compromised one. Hence if you really do not have the space for a centre speaker (and an inch high is not really big enough IMHO) then you may be better just upgrading the front L/R instead and not having a centre at all. Something like the Wharfedale diamond 9.1 would be a good option.