Anyway there are instances where wholesale companies send you some items that you can test with, especially if you are going to pay for the shipment. If you are ordering offshore, try to ask how the fabric substrate is made which would include the materials and processes used. In addition, remember that the lowest price is not always the best.
The first thing to do is to determine your target market. Are you going for graphic designs on mouse pads? Try checking out that competition in that category. See what big Asian companies are marketing mouse pads for. They are going to be your main competitors. In case you'll steer away from them and target the smaller market within decent margins, study the local competition for the said product. Be sure to know what clients are going to benefit for the product itself.
If you'll go with mugs, check again with the Asian market, especially China, to see prices and how the mugs are being sold. If you find the rate to be outrageously low, look over the local market. If it's okay with you to personally contact targeted clients in the local area, you can create your own market of clients who probably don't want to purchase loads of boxes of box, but may order a few hundred dozens. You might be able to compete in that locally targeted market. And then you'll require a really good and dye sublimation printing machine. It would depend on the product and target market you are looking at. If for mugs or mouse pads, you'll want the smaller printer. If for large format displays like banners, you'll need a wide format printer.