First stop: dollar store and goodwill (or a similar thrift store). You can get cheap candle holders, little boxes, and offering bowls. You can even find bottles and jars. Sometimes thrift stores have nice framed paintings and little statues. Also look for vases to put flowers in, incense trays, placemats that you can use as altar cloths, ect. You can find something kind of like you want, stick a new paint job on it or some paper mâché decorations, and have the perfect piece for a couple bucks.
Second stop: paper mâché and polymer clay. Super easy and cost effective ways to sculpt anything and everything. Make statues, bowls, decorate plain candle holders, ect. You can also experiment with cinnamon clay, cornstarch clay, and air dry porcelain. Look up tutorials online.
Third stop: the outdoors. Find pine cones, dry wood for wands or knife handles, get creative. Get some dirt or sand, put it in a bowl, and stick some incense stick in it. Instant incense holder. The dirt will also hold the heat from charcoal discs to burn loose incense.
Final stop: craft stores. Fabric remnants for altar cloths and pouches, silk flowers, and whatever else you need to put the finishing touches on your altar.