I am looking for guidance on hardy small plants/bushes suitable for a gravesite. I want to plant something that will stay small
To your requirement that the plants or shrubs are hardy in Newtown, Pennsylvania, I would add that they require minimum maintenance and are attractive year around. Also, some cemetaries have rules regarding what can be planted and where.
Shrubs are usually large, even if they are said to be dwarf. You need to look for miniatures. Chamaecyparus conifers are attractive year around. You might look for C. obtusa 'Minima' which grows slowly and tops out at 16". Many conifers can be kept small by pruning. Azaleas are often evergreen and you may be able to locate miniature varities such as Rhododendron 'Alexander'. A nursery such as Shady Brook should be able to help you.
Ground covers are often evergreen and grow only a few inches high. Some of them spread out a bit and may spread into the grass. Sedums are goods candidates for your site. Sempervivums (Hen and Chicks) would work well and spread very slowly. Ajuga varieties have very attractive foliage but may spread or move into the grass depending upon the site. They tend to move towards the sun over the years. There are miniature hostas that would fill the bill except that they aren't evergreen. There are newly developed Heucheras (Coral Bells) that are dwarf and may be evergreen in mild winters. Russell's Wholesale in Richboro have outstanding dwarf plants in these categories at modest prices and will sell to the consumer. They do not carry shrubs.