Most ISA network cards are for the 16-bit bus, XT class machines only have 8-bit slots. This doesn't necessarily mean that the cards will not function, but you also need a packet driver and they very often use 186/286 instructions and will not run at all on an 8088.

The Intel EtherExpress 8∕16 will work, so will the 3Com 3C501. You may also have some luck with generic NE1000/NE2000 clones.