Where can I find a refurbished iPad for under $150?

About the only place you're going to find one that cheap that is guaranteed not to be activation locked is at a place like Gazelle.

https://buy.gazelle.com/collections/ipads

They won't always be in the greatest shape (you pay less for a beat up device, more for one that isn't).

Also, if you're looking for one with cellular capability, watch carefully for ones that are locked to a carrier (Verizon, AT&T, etc.) as opposed to one that is unlocked (you can use it with any carrier you want). But one with cellular is unlikely to be $150 and under.

Current choices are very slim at your price range. Only an iPad 6 is available. iPad 7's and 8's are in your range, but they currently have none in stock.

https://buy.gazelle.com/collections/ipads?sort_by=best-selling&filter.p.m.custom.carrier=Unlocked&filter.v.price.gte=0&filter.v.price.lte=150

I wouldn't expect one that would be of any use. We can only direct you to the Refurbished products provided by Apple. These come with a brand new battery and also a warranty with nothing in that price range.

Refurbished iPad Deals - Apple

You can certainly check with any other retailer. You may also want to consider if they are going to need a new battery that will be an additional expense, which is usually about $100, and if they offer a warranty at all.

The one thing you also want to consider is the risk of purchasing an iPad from a third party or unauthorized Apple Seller.

The All Too Common SAD Reality of Buying … - Apple Community