A Wiccan wedding is still a wedding. The young couple are just starting out and will need all the basic household goods any couple starting out will need. Cash or gift cards are also a very well received gift to the new couple.
If you want to get something special for the couple beyond basic wedding gifts you can get them something special to bless their house with. A lovely wreath or decorated broom that they can hang on their door or a wall in their home maybe is an example of the direction you can go in. Hand made (or someone else's handy work if you are not the crafty type) is always a wonderful gift.
If you know them well enough and know if they have a particular deity they strongly associate with you can find them a lovely statue of that deity that they can place in their new home. There are some beautiful ones that can be found in a very reasonable price range.
But in all honesty, unless you are super close and really know of the faith, buying anything along the lines of altar items, which is the impression I am getting from your post, is not very easy.
With all of that said, anything personal that you would give to any new couple is absolutely perfect. What is far more important is the intent of giving the carefully chosen gift more than what the actual gift is.
Thinking back nearly 20 years, we got some gorgeous candle holders, our wedding invitation beautifully framed, a hand quilted covered photo album, an amazing decorative plate bought at a renaissance faire by a friend of ours, we also got dinner plates, drinking glasses, lots of gift cards and cash that we pooled together to put some furniture in our tiny one bedroom apartment and finally buy ourselves a 4 slice toaster (what a luxury! LOL). Everything from the blender (because we did get one) to the more personal gifts were all received with love in our hearts from those people we loved and wanted to share our special moment with.
Buy a gift from the heart and give it with love and it will be perfect. That is all the Wiccan faith desires.