does anyone know if there are any stretching exercises or techniques I can use to improve this?
There's a very simple stretch that involves simply pulling the thumb backwards. You just have to be careful not to overdo it or force anything. Apparently De Larrocha was obsessive and did thumb stretches constantly. Some pianists have thumbs that can extend almost perfectly sideways. I'm nowhere near that, but have gone from a 10th to a very comfortable 11th (I can span a 12th if I block two notes with the thumb- not that it's any great use). Also, playing chords that don't stretch you to your full limit and squeezing slightly inwards (not outwards) against the side of the keys generates a reaction force that can actually open the hand out more. Again, you don't want to go crazy though.