I write this letter to tell you that I need to buy of English Test book,Because I don’t find of this boosin internet.
buy是及物动词,不需要介词of.
because作为连词使用,而且前面是逗号,因此首字母小写.
"find of" 更多的是“发现”的意思,但是上下文感觉这里需要的是“找到”的意思,直接用“find”就好.
“this boosin”可能打字打错了应该是“this book in”吧
其他关于时态问题,因为是正在写所以最好前半句用现在进行时,而没有找到书是之前的事情所以最好用过去式.
这句这样改会比较好：I am writting this letter because I didn't find the English Test book that I really need on internet.
I hear of I can look forthis book in your bookshop.So I hope you can help .I want to know about thisbook cost and size.You can sent me by EMS .
herd后面跟的是一整句所以是从句,不能用介词of,而是应该用that.
look for是寻找,不是找到,在这里意思不是很合适.
改成这样比较好：I heard that this book is available in your store.Could you please find it for me?I also want to know the cost and the size of the book.If you find the book,please deliver it by EMS.
I will thank you very much
这个：I will really appreciate that.或者直接 Thank you very much.