What types of things are you looking for? You will find a lot more inexpensive items on HipStamp versus
ebay because HipStamp has no listing fees for most items, they only charge a commission when an item sells (which is also lower on HipStamp than
ebay). I have bought lots of inexpensive items (5 cents to 10 cents, sometimes less) with free shipping on additional items. You may pay $1.00 or more shipping on the first item so it only makes sense if you are buying dozens of cheap stamps at the same time from the same seller. But do check their postage fees, which is easy to do, some will charge like 79 cents per item regardless of the cost or the number of items. Others offer a flat rate for shipping no matter how many items.
Some examples of sellers of inexpensive US stamps include AworldOfStamps, okstamps, L&S USA Stamps, tatumfan, stampbundle. I have no association with any of these except that I have had some successful purchases from at least some of them.
I second Henry Gitner as a high quality online dealer with a large stock, but I wouldn't put them in the category of inexpensive. Sometimes to fill a space you need to pay full catalog and hgitner may be the only one who has the item.
I would add Bardo Stamps as another good online dealer with a big selection and a great web site, including many minor varieties. Again you will probably pay 25 cents for a stamp that catalogs 25 cents but if you need to fill the space, it may be worth your time to buy everything under one roof rather than hunting down multiple sellers and paying 10 cents each for 10 different stamps...
Check for some smaller local stamp shows as well. The dealers at the big shows won't have a lot of the modern inexpensive stamps in their stock but the smaller shows will.
Have fun filling those spaces!