Maybe I missed it, but do we know whether you're putting down ceramic tile or vinyl? (or maybe something in between?)
The "home decor" shows on late night TV have been showing - and re-showing - a demonstration on laying down ceramic tile and/or wood laminate in a couple of different situations. They advocated using a "leveling compound" that's stirred to a thin slurry and flows to level. I didn't make note of product names but more than one kind was mentioned. Your local lumber yard should recognize what the stuff is.
At least one of the compositions used didn't require mixing (all in one can), but stirring was recommended, and I think a couple called for adding water to thin.
I would expect that on a wood subfloor a "barrier membrane" would be advised before applying a liquid leveler, but requirements shouldn't be too critical, and I don't really know whether one is recommended. I believe they poured the leveller directly onto a concrete floor in at least one case.
I think the ceramic tile was laid directly on the leveler material, but for a wood laminate floor they added a "pad board" under the wood.
For vinyl tile, the usual thing is to just lay down an "underlayment board." If the dips and dents are fairly small, a HARD TEMPERED Masonite specifically called "underlayment board" about 1/4" or 3/8" thick may be recommended, but for bigger gaps a thicker high-resin chipboard composition is usual, and is available in thicknesses from about 5/8" and up.
The Masonite-like underlayment is very hard and the last time I put some down I probably bent about as many nails as I managed to drive through straight on a first try. (The recommended Underlayment Nails are hardened and ribbed, usually, and sometimes break instead of bending.)
The thicker "looks like chipboard" stuff, because of the high resin content is "very dense" and a 4 ft x 8 ft sheet of 5/8" thick (a popular size) may weigh 60 - 80 lb or more. (I don't like to carry more than one end at a time, so I'm always havin' to go back and bring the other end along.) Since it raises the floor level, you will want to consider whether all the cabinet doors and drawers will still work.
It may not be the best place to get your stuff, but if there's one nearby Home Depot probably will have a class - or make one up - where you can get the real poop on what you want to do. Although you do run into people who have more opinions than skill just about anywhere, I have found that I don't have to explain near as many things there as at some nominally comparable material sellers.
After I've convinced the sales people that they really do have what I'm looking for, and make them dig it out, I get a lot of "oh, so that's what that's for" just about everywhere I try to buy stuff. I guess it goes with being "experienced."(?)
n.b. I also learned recently that Home Depot will give military and honorably discharged veterans a 10% discount, but you do have to show them your discharge papers each time you ask for the knockdown. (I assume that's just US military and vets.)