If I use an 8 speed bar end shifter with the TT bar on my Allez, will I need to change any other parts of the drive chain? My current bicycle is a 2013 Specialized Allez C2. It has an 8 speed rear cassette. I'd like to put in a TT handlebar. It's a given that I'll have to change the brake levers, as well as the shifters and associated cables. Now, I spoke with a repair guy at Cynergy Cycles, and he said that there are no 8 speed front end bar shifters, so pretty much the whole drive chain would have to be replaced… I looked it up and found only one: a Shimano Ultegra SL-BS64 Shifter. Would that pretty much do the job? or would anything else need to be changed as well?
Pretty much any 8 speed Shimano-compatible bar end shifter will work if you're running a Shimano drive train (If you're running SRAM, you'd obviously need a SRAM compatible one). 

Those will work, as will the ones from Microshift 8 speed among others. You'll need to re-run your cables, obviously, though.