How can I repot a cactus without buying expensive cactus soil?

Here's a DIY recipe for cactus soil that might be helpful

https://www.hometalk.com/diy/grow/plants/diy-succulent-cactus-soil-mix-43281982

Hi Donita, here is a link to a good recipe to make your own cactus soil.

https://getbusygardening.com/diy-succulent-potting-soil/

Cacti will grow in any soil as long as the clay/terra pot has good drainage holes on bottom and you water it accordingly to type of soil it is. ones that hold water like clay and sand alot less of course. I have seen cacti growing out of a crevice in rocks as long as there is alittle bit of soil,sand,organic matter for it to get a foothold. Why do you think they need repotted? Cacti can grow in same pot for years because they do not have large root systems,and do better when they are less disturbed and root bound. Because the tops are big doesn't mean the roots/cacti need transplanting,just means it's doing really well. the less you mess with them the more chance they will bloom,if the roots are growing in large pot it most likely won't bloom as readily since energy is going to root growth. here is good recipe... A pinch of rock dust

One part coarse sand – I use builder sand

Four parts bagged potting soil – like an African Violet mix/good organic material

Five parts perlite (not vermiculite)

online info.----- It’s a must that your pots (I prefer clay pots) have working drain holes and that there’s no water sitting in the bottom. Mix in some organic material to keep the soil mix from quickly drying out. The roots love an airy, well-draining soil mixture that dries and gets wet easily.

You must prepare for repotting as soon as you see the cactus’ root system begins exiting through the drainage holes in the bottom. Most species of cactus and succulents need repotting at least once in every 2 to 3 years.

Repot the slower-growing types every 3 to 4 years. Repot in the spring season when they begin actively growing. It’s recommended to water two days before to ensure the roots and root ball are moist.

As much as gardeners will cringe, I have used plain old Miracle Gro potting soil. Worked out just fine. Cactus is getting huge. If you want a photo, let me know!