Handheld vacs are a great source of Sci fi shapes, just make sure its made from ABS and it will work out fine. Many of the cheaper ones are now made from HDPE ( high desnsity polyethylene) which is not conducive to good modelmaking no matter how hard you try.
What I hope to show in my builds is that we need not be reliant on kit manufacturers for a satisfying and creative hobby. Nor do we need to wait on the next movie for inspiration. Using found objects is a way to build something original without having to start from nothing, or a blank sheet. It has inherent limitations in what is possible due to the existing shape and it is limitations which can be the foundation for "great" art. Doing something original in this way frees you from the stress of accuracy to a subject. Nobody can tell you you got the shade of grey wrong, or the wrong kit part or the panel line should be 2mm to the left. It can be a lot cheaper too as you do not need any specific and rare donor kits for parts, and the found objects are generally free or very cheap from thrift stores or dollar stores to use the American terms.
Any model kit will do and I only buy them when they are cheap or on special as they are ludicrously expensive these days, particularly in Australia, where a Tamiya tank kit can be a whopping 40-50 aussie dollars. My local toy shop had them on special for $25 before Christmas but annoyingly only had 1 of each model. I always like to buy two of the same model because you invariably need at least two of the same part for a symmetrical model.