Look locally for a needlework shop who will work with you to get what you want. We have a lovely one where I live, and even if she doesn’t carry exactly what I need, she will get it.
My personal favorites are Mirabilia patterns, which you can see online at http://www.mirabilia.com. I have done 4 big ones, have one on my frame, one waiting for me to finish, and 1 small one for the birth of my newphew, all Mirabilia.
Also look at All Stitched Up http://www.allstitchedup.com, The Stitchery http://www.thestitchery.com.
I will warn you, if you like Theresa Wentzler’s patterns, but haven’t done one before, be prepared for madness. I picked out a pattern I loved, and checked the colors list as I was walking to the threads aisle. it had about 100 colors listed, and every single one of them was blended. The actual number of skeins to buy was something like 60, but they were all blended. eeek.
I have tried PCStitch, but it was several years ago, so it may have improved. I wasn’t impressed. I took a stock photo of a lighthouse from print shop, and it converted it to a pattern. That was pretty cool, but I didn’t like that what I would have stitched as line it rendered as actual cross stitches.
Last but not least, if you are a serious stitcher, and you have the money to splurge, buy yourself a decent floor standing frame. I have one that a local artisan makes and sells which uses scroll type frames, and supports the frame on both sides, which was very important to me. It sped up my stitching, because I was able to have one hand on the top, one hand under the fabric, and it goes really fast.
I forgot, also, buy a good lamp to use while stitching. I have an Ott-lite now, but I used to use a swing arm lamp that I could shine directly on the fabric.