DIY is a great idea, especially when you enlist your friends and family. First ask if anyone you know already has items you can borrow. Then yes check FB Marketplace I have found so many free items that way. Check flyers for items you want. Michaels can be expensive but they usually have 30-40% coupons, I used a discount coupon every week for several weeks until I had purchased all of my submersible lights. And always walk thru every store's clearance section. Use what you have already, jars, bottles, vases, tablecloths etc. Make fun project days with your helpers to make it easier. Remember to have space set aside to store your creations. Also remember to ensure you have a plan to get your creations to and from the venue (or with you at the hotel you are at) or enlist others to go to your place to pick them up. Have fun!
For our centerpieces im doing spray painted wine bottles (having friends and family save their empties for me) then we are using fake flowers (I have gotten some from Michael's when they have their 60% off sale & dollar tree has some nice fake flowers sometimes)
I've also used a lot of dollar tree picture frames for decor/signage!
There are local wedding buy and sell groups on Facebook where people sell stuff from their wedding, definitely more cost effective since they are used! And often in really good condition since they would have been used once.