You do not need tb500 or a flex bar to treat tennis elbow (or golfers elbow) I have treated it myself this is exactly what you need to do, firstly stop working upper body until the pain is gone second there are two stretches you need to do I suggest doing them with both arms because typically tennis elbow on one side if the body is caused by a weakness in either shoulder elbow or wrist, for me it's my wrists
Anyway the stretches are so simple you probably won't think they work but they do, first one arm at a time lets assume the pain is in your right elbow stick your right hand straight in front of you palm facing down and make a fist. This probably hurts but that's OK. Keeping a tight clenched fist grab your fist with your left hand and bend it down towards your feet, you will feel a pronounced stretch along the top of your forearm. Do not over exaggerate the stretch just get a good stretch and hold that for a while. 20 seconds minimum, you can hold the stretch as long as you like. Do this as many times a day as you want, the more the better. Repeat with other arm, stretching left forearm.
Stretch two is the opposite, so palm facing down again open palm so that your hand is flat fingers straight and with your other hand grab your fingers and pull back so you get a stretch on the other side if your forearm. Same deal hold for a while and do the other arm.
You should see results in a couple days. Stop all offending movements until the pain goes away. You can mask pain with advil but don't go to the gym as the advil will mask pain which you don't want. This is not something you just train through. Advil reduces inflammation and speeds healing, we want that.
You can get epistraps at Walgreens they help 5% in my experience. The stretches 100% work. The underlying issue you should address eventually I'm guessing it's either wrist weakness or some imbalance in your shoulder complex. If you train heavy but never do wrist curls, reverse wrist curls etc like me you likely have an imbalance in that area. Biceps are stronger than what your wrists can handle or something like that. Train wrists with arms or after back once the pain goes away.
It may take a week to see dramatic results so be a little patient. Do those stretches at least three times a day try to hold the stretch at least 45 seconds at a time. You can also ice your elbow but that's only if you are also doing other things as icing in my experience is not effective by itself.