You can delete your Ultrapeers one by one. Choose the one you've been connected to the longest, select it choose Remove. It may take from a few seconds to a few mins to get a replacement ultrapeer to stabilize your connection. Then see if searches have improved. If not, remove the next longest ultrapeer on the list. Some people might just remove all at once, but that's up to you. Not all of them will be bad I'm sure.

As for adding ultrapeers manually, you can use the gnutella.net file. You can find one to download from this page 3/4 way down the list: If LW 4 Will Not Connect (click on the blue link). This file can be opened with Wordpad or any intelligent wordprocessor.