Cheap 'n cheerful approach:

Visit your nearest decorator's merchants (e.g. branches of Dixons (and I don't mean the telly shop!) here in the Midlands). A few quid spent will give you a lifetime's supply! Alternatively you can buy walnut treatment oil from your local gunshop, but ounce for ounce you will pay ten times as much for what is essentially the same thing. Also get some fine/extra fine steel wool.

Optional step: if your stock is brand spanking new and feels a little rough, take the action out of the stock and smooth with the steel wool. You may also want to pick up any awkward grain with gentle steaming from a kettle before smoothing. Ensure that the stock is left to dry thoroughly (not on a radiator) at least overnight before proceeding...

Get some boiled linseed oil and terebine dryers. Find an old 35mm film canister or similar and 1/2 fill it with boiled linseed oil. Add ~ 1/2 teaspoon terebine dryers to that amount (sorry not exactly scientific measures here!)). Stir well.

Put a couple of drops of the mixture in your hand and rub very thinly and vigourously with your hand into the stock to generate some warmth. Once the whole stock has been thusly treated (remember very sparingly), leave it overnight and repeat. Do this for a week. Ensure that each coat is thoroughly gone off before applying the next. Thereafter, treat in this way every week for a month or two.

It should look pretty darned good after this. There are more elaborate kits that will produce even better results, I'm told (heard good things about kits from Chambers and Napier).