My son has lost his 2 leaf insects the day before Christmas eve

he was quite upset, he has now asked for a spider

I have had a chiliean rose before that I had for 14 years so I know most of there habits but does a red knee differ alot ? The reason for asking is that I kept my spider on sand & never had a problem, in fact when I changed substrate & she didnt walk on the floor untill I changed back to sand !!!
I would love to get a spiderling to watch it grow ( Im now sounding as if its for me lol) WE would love to watch it grow ! but is the success rate good ?
Any info would be great , even better if you can tell me were I can get one cheap