Can I buy the Lcd and digitilizer separately and put them together?

Yes you can. I don't believe it takes any special skills or tools. I believe it is more of a mind set that you are going to do this in a professional manner and not take short cuts. Wear powder free rubber gloves. Use your framing to base things from. Put your digitizer in it. Use canned air and a brush to remove any dust or particles. You need to use reflective light to find the dust and particles if there is any, on either part. I recommend using double sided tape for the adhesive. 3m puts out some good tape for this purpose. It is very thin and acts like a glue after pressure is applied. There is a reference to it here on ifixit somewhere I purchased 100 yards of it a year ago, for like $10 - one of the best purchases I've ever made. Put the tape on the lcd part. Mate them together with the digitizer in the frame. I've done hundreds of digitizer to lcd matings like this. If you aren't willing to approach the task like a professional and attempt to come as close to factory as humanly possible then oldturkey is correct you need to hang it up and buy factory. Again I believe it is more of mind set than skills or special tools.

Jade here is a reputable seller selling the iphone 4g Retina display LCD. Many of the sellers out there are selling a knock off TFT LCD. The link I gave you is on Amazon. If you do a google on the seller name you will find their WEB site.

Here is a link to the double sided tape.