does anyone else have this problem and do you have any advice?

I'm a 36FF and find it difficult to do cardio, I've heard the elliptical works wonders but I can't afford to go to a gym or buy home equiptment It's driving me mental I'm trying my best with sit ups, push ups, leg lifts etc but don't feel fully accomplished as my exercises are very limited, and I've tried getting a good sports bra so that doesn't work, any advice?

Get a better sports bra. Or two. I was running when I was an H/I cup.

I generally aim for proper band size and go a bit smaller on the cup size for extra compression (and because nobody sells 34 DDDD). You can do that as long as it isnt underwire. Then if your cups runneth over you may want to top it with a general uniboob compression bra.

Also, you can find old reebok steps easy used. Stepping up and down on them is very low impact.

One thing I've done (I'm a 38 G) is wear a regular (non-underwire) bra (old nursing bras for me LOL ) and then put a sports bra (or two) over the top.

It's not ideal, but I can't afford a "good" bra in my size. So it holds the girls down so I can run/exercise somewhat normally. (I just can't do jumping jacks...)

I can't do jumping jacks - I wet myself - lol!!

Enell Sports bras! I was a 34 G before my breast reduction and they are wonderful. You can find second quality ones on eBay.