IDT offers a 5' Thiol modification with a 6-carbon linker and a 3' Thiol modification with a 3-carbon linker. If you would like a longer spacer arm, then you could order an oligo with a terminal (5' or 3') Thiol modification as well as an internal spacer modification immediately internal to the Thiol modification. For example, here are two sequences, with and without the spacer (I used a "spacer-18" for this example, although we offer numerous kinds spacers):
5' /Thiol-C6/ACTGACGTACGTACGT 3'
5' /Thiol-C6//Spacer18/ACGTACGTACGTACGT 3'
You are not limited to incorporating only one spacer in your terminally modified oligo, you could also design the sequence with multiple internal spacers for even longer separation. Please see our Spacers Modifications Catalog Page for more information.