If you’re looking for a VNA, I purchased the new Plus4 model as my first, and I’ve been very impressed with it: https://www.tindie.com/products/hcxqsgroup/4-nanovna-v2-plus4/ Sign up to be alerted when they’re back in stock, and pounce when you get the email - they sell out quick!
Honestly, a nanoVNA may be the best investment, as it allows you to test/tweak your own antenna builds in terms of SWR - I found that one of the antennas that came with one of my T-Beams was horrible and losing around 3/4 of the power sent to it! If you’re looking for an inexpensive off-the-shelf high-gain rubber ducky omni antenna for 915MHz in the US, I’ve been quite happy with the Superbat that @jefish noted in this thread: Antenna improved range - #62 by eriktheV-king