i *think* HOP math is not sold by individual grade, but rather it covers K-2 (or K-3??), so you would buy one big box, ykwim? i'm not sure though, because i've only purchased their reading. you can also buy their workbooks which cover one grade only, and they're cheap and pretty good. however, i would NOT buy from their website. it is SOOOO much cheaper to buy from target ...or homeschoolclassifieds ...or amazon. anything but the HOP website. here's what i found at amazon. hth!
http://www.amazon.com/s/ref=nb_sb_no...n+phonics+math
http://www.amazon.com/s/ref=nb_sb_no...n+phonics+math