As you might know, there is a strike going on in France, which affects all TGV TER trains, including some buses run by SNCF. SO I am looking for an alternative travel option from Grenoble to Valence by longdistance bus, if there is any.
SNCF Rhône-Alpes runs TER line 61 between Grenoble and Valence; the trip takes a little over an hour and the trains run approximately hourly. There are also buses, but they go to Valence TGV far outside the city; you then have to take another bus into the city.
Due to the strike, there is limited service. At the moment few of the trains are announced to be running but there will be substitution buses for some of the services continuing to Valence-Ville after Valence-TGV. So you can probably make your trip (except if you were going to take one of the evening services), but allow for some delay.
Grenoble and Valence are in the same région but in different département and have a train connection. In such cases, it's rare to have non-SNCF buses. I don't have authoritative knowledge of this region, but the regional council website doesn't list any likely-looking bus lines, nor do any of the département-area sites for Isère and Drôme. While it's possible that a combination of shorter-hop buses could get you from here to there, it's likely to end up taking all day (many of these buses may have no midday service, and they often wouldn't wait for connections) — even if a connection exists in theory, there'd be a risk that you'd have to hitch-hike some of the way (which, incidentally, is a valid option if you aren't in a hurry). Rome2rio only mentions the train and the bus via Valence TGV (granted, I don't know how good their coverage of regional buses is).
So take the train or the bustitution.