Just get the regular cinderblocks. That's what I used. They should be less than $2.00 each at Home Depot. And, you can plan marigolds or other small plants in the holes.
Not only that, you can take 'em with you if/when you move.
How many do you need for a fairly standard bed? Are they a foot long?
The moveability is a plus, you're right. Hadn't considered that. I wouldn't be spending on something I'd have to leave behind. And I could probably get 'em in the van if I wanted to buy 'em.
While you're here...what sorta mix do you put in a raised bed?