Question....I shoot baseball for a hobby and need to get out as cheap as possible. Should I upgrade my lens for my Canon Rebel T3i or purchase a 90D?
I am not able to upgrade camera and lens at the same time. I'm not even sure if there is another lens that would fit the Rebel.
I currently use the EF 75-300mm 1:4-5.6 III and my images are just not as crisp as I'd like.
" I'm not even sure if there is another lens that would fit the Rebel."
Any lens that fits your Rebel T3i will work on a 90D.
"I shoot baseball for a hobby and need to get out as cheap as possible."
That is usually not a good scheme to get great photos. You can increase your "good shot" chances by using Raw file format and a good post editor. Raw format is free. The very good from Canon DPP4 editor is also free.
I agree with Ernie's comment that you will get the best results by upgrading the lens. The 75-300 series is arguably Canon's worst optic. If you want to get much better results, I would suggest going for the 70-300 lens of which there are two candidates available, the MkI and MkII. See my review of these here: Canon 70-300mm Lenses.
Canon have a special deal on the MkI version of this that should hopefully not break the bank, and as I noted in my review, this lens can take great images, but you need to act quickly - see: https://shop.usa.canon.com/shop/en/catalog/ef-70-300mm-f-4-56-is-usm-refurbished
I agree with the other responses...
Your first priority should be a better lens. That will do much more for your images than a camera upgrade.
In addition to better optical quality, you also can get faster focusing with a USM lens and sharper shots thanks to Image Stabilization.
The EF 70-300mm IS USM lenses are good. There are two reasonably priced options in the original and the "II", as well as a more advanced "L" that has a bigger price tag. The "L" also can optionally be fitted with a tripod mounting collar, which can be handy at times. .
Because baseball has a large field of play, even better would be one of the two versions of Canon EF 100-400mm L IS USM. The "II" version is particularly good, if rather pricey. Shop refurbished or used, if need be.
Later you may wish to upgrade to the 90D, too. Its more advanced AF system and 10 frames per second shooting rate would be handy for sports photography. With nearly twice the resolution of your T3i, the 90D would also allow for a bit more cropping, if needed. It also would give you the option to use a 1.4X teleconverter on either of the 100-400mm lenses, for an effective 140-560mm combo for tighter shots of those outfielders in action. (You would lose autofocus fitting a 1.4X to the lens on your T3i.... so that's a no-go.) I can tell you from personal experience that the Canon EF 1.4X II and III Extenders both work extremely well with the EF 100-400mm "II".
EF 100-400mm L IS USM "II" + EF 1.4X "II" at 560mm, f/8 on Canon 7D Mark II, ISO 1600, 1/800 hand held...