Horrible temptation befalls me!
Doing a quick Google Search to see of there are any affordable cushions like those featured in the OVA/OAD, I see that the Crunchyroll store has a pre-order up for the first part of season one. That Time I Got Reincarnated As A Slime is also supposed to return for a second season next year, I believe. Plenty of time to get discs of season one, probably.
By the time you factor in shipping costs it also looks like the Crunchyroll store's plushie would be cheaper than Amazon. Although I'm kinda afraid the dogs would claim it as their personal pillow....lol.