I am the queen of not having enough money to afford brand-name professional clothes, and also the queen of not wanting to pay high prices on said clothing even when I do have the money. Here's some ways that I've managed to find good quality work clothing as inexpensively as possible!
Shop sites that look somewhat sketchy, such as shein.com. The key here is to READ. THE. REVIEWS. I don't even consider buying an item if it doesn't have at LEAST 500 reviews. People who buy clothing from these sites are usually pretty honest - if multiple people are saying the material is cheap and wrinkles easily, trust those people. Sizing is usually a crapshoot on these sites, too, so look at measurements for the specific item if they are provided, or again - trust the reviews. I generally go a size up anyway since I can belt or tuck a shirt that's a bit too big, but I can't make a shirt that's too small fit nicely.
Thrift stores. I know some people have some feelings about thrift stores, but I honestly don't think they're that big of a deal - just wash the clothes you buy, and it's no different than a hand-me-down from a family member. The key here is to make sure that you either 1) try items on in the store, or 2) the store permits returns.
Thredup. This is an online subscription-based box service that sends items on a regular basis (you pick the intervals! Their most popular are monthly or every three months). You try the items on, keep what you like, and send the rest back. Thredup is a great option for people who don't have a ton of time to go shopping, or who just don't enjoy it very much. I have a couple of tops from Thredup, but since I LOVE shopping, I enjoy making time for it even when there's more time-efficient options.
Stores that don't technically qualify as "professional." Target, Old Navy, GAP, H&M, and the like all have things like dress pants, blazers, blouses, dresses, etc. A blazer's a blazer, dude. Especially in the Zoom Era, no one's going to be able to tell that your top is from Forever 21 and not Banana Republic.
I'd also make use of your friends - do clothing swaps every month or so to freshen up your wardrobe! I know this won't work for everyyyybody, but I think it's a really easy way to rotate some options through your closet without breaking the bank.
Got other ideas? Let me know!